Your problem is meta, solve it meta

Tell other players, including your DM, that you are planning to play this character that does justice serial-killing. Other characters might feel this character is shady, which is fine, but ask them not to uncover your hobby too quickly. Tell them that you want to develop this character, at least for x sessions.

The in-universe explanation would be that most people tend not to stick their nose in other's business, and respect each other doing in their time. Ask for your DM's cooperation.

• If they try to follow you, DM will say they lose you after a while (you should be trained in shaking pursuers, I guess?)


• If they become suspicious of you, ask the player to explain why their character became suspicious and what they are suspicious about. Everyone has secrets to hide, and they won't know exactly what you are doing


• Start stating where you will be hunting your prey after others state their whereabout. You can choose victims where other PCs are not there


• Basically, whatever they want to do, you always escape and no one knows/confronts you until the time to do so, which is when you say so

DnD is not a game of hide and seek, a competition where other players want to uncover your dark side, unless you are playing the villain. Even so, DnD is a game of story. What fun it would be if the villain is uncovered on first session and killed? Instead, ask their help to build the story, until the day where you think you want to play another character, when their suspicion is proven and this time you fail to escape and must accept the judgement.

Develop a code with your DM

I've played a similar character before: one that was delusional and paranoid, and became quickly convinced that random passerbys were part of a vast and sinister conspiracy against him. My GM knew all this about my character, and we developed a particular way of describing my actions. In my case I said that I was "questioning" the person (my code for murdering them), and would give very vague descriptors of what that meant. For example:

• "I question this person quickly: I am concerned that their roommate
  may return soon."




• "I question this person thoroughly, but make sure to
  do so somewhere out of the way."

The GM knew what I was saying, and would give similarly vague and brief descriptions of how that went down. Such as:

• "Ok, you succeed."


• "It's not without complications, but nothing of consequence goes wrong."

The descriptors were sufficiently vague that other players didn't feel an immediate need to be a part of that interaction, but it also allowed me to gradually build up the perception (on the part of the other players) that something peculiar was going on (for example, at one point my GM let me know that my target didn't speak any languages I spoke: I said "I don't see how that's relevant. I question them.").

My other piece of advice would be to be careful that you either keep these interactions brief (in real time), or give other characters an opportunity to interact with you (e.g. have them see you stumble out of a bar without knowing that you are currently stalking a target, and either make an excuse to go about your business or [perhaps even better] divert from you plan and spend some time with them to prevent them from being suspicious). A scene that only has you in it is likely to be less entertaining for other players. Besides, the thrill of almost being discovered could add an interesting spice to your element of the story.

It sounds like what you need is a bit of solo play.

You're trying to turn your character into the villain of an ongoing mystery side-quest, based on the fact that he goes around murdering people during downtime. Okay. You want to play out the "murdering people during downtime" part. Cool. You don't want your fellow players to react to the inevitable metagame knowledge. Cool.

So why are you doing it in front of them?

You don't need codes (potentially suspicious). You don't need note-passing (always suspicious). Any time you're in front of the rest of the party, you wouldn't be doing this stuff anyway. You just need to get together with the DM outside of the scheduled group times (possibly handled via chat, phone, email, or whatever) and run through the murder scenes (perhaps in a somewhat abbreviated manner). Then the next time the party convenes, the DM talks about the horrible murders that occurred when everyone was on downtime. They'll almost certainly investigate in-character (its an obvious plot hook, after all) but that's what you want, right?

It's been a few years, but I did once run a campaign (of heavily modified D&D 2nd edition) that involved a significant amount of meeting separately with specific players in ones or twos to run personalized scenes and then pulling them back together for the group events. It required a fairly heavy DM investment (something I had time for at the time) and it meant that the party never really gelled as a group, but it worked quite well at allowing differences in player knowledge, and sometimes-antagonistic interactions. Your case is a much more limited scope. In your case, I would expect that it might lead to some emotional separation in your mind between you and the rest of the party (which, honestly, seems appropriate given what you've said) and some additional effort on the part of your DM (but not an overwhelming amount). If the DM is not willing to make the additional investment, this technique is not for you. If they are, though, I believe it would work well for you.

Do they need to know?

The first question you should ask yourself is, do the other players need to know that you play Dr. Jekyll/Mr. Hyde? If not, then you need to find a way to tell your GM what you do and when you do it.

Leaving the others in the dark

There is a quite simple way to get info past the others without them knowing, and that is writing a quick note passed over. You might use text messages/WhatsApp or something similar if you have relaxed table rules for such things. Best combine it with some sort of misdirection.

This method to inform the GM (or just other players) of some plot you are doing works best if everybody does this and uses it for even trivial stuff.

For example, the Paladin-Player hands a note "I want to help the poor woman A to find her husband" to his Cleric friend, they nod, making that a whispered exchange before they announce their actions to the group. The Rogue meanwhile might announce something (see the anecdote) and do something entirely different to build up plots and gears to work later.

A big caveat though: This method works better in groups that are ok with some intrigue and side-quests and is best answered by the GM to also use the method to distribute information. This increases the chance to play the character and helps with the distinction of Player/Character information.

Anecdote

This is what I did once to keep the other players in the dark, more for the comical effect - and to prevent all a loot debate who would benefit most:

R[ogue]: hands a note "I will stick the healing Gem into the Paladins Pocket" to the GM "Finders Keepers, I don't tell the others and pocket it."

GM: nods "OK, can I see your sheet a moment? Hmmm... Roll against 12... Your Mod is +2"

R: clattering dice "13, got it."

GM: "You succeed"

[3 hours later, the paladin falls to the ground with 0 HP]

P[aladin]: "Ok, did my job. You finish the Dragon for me..."

R: "I don't think you are out of combat yet. That healing gem..."

P: "The one you stole and our PCs don't know about?"

R: points to the slip of paper "Yes, that one. It's in your pocket."

GM: nods

What I did

I am currently playing a similar character, except mine is a secret operative for the other side of the war with orders to assassinate a particular officer, rather than a serial killer. I took the approach of creating an alter-ego for my cover RP, and also creating a "real" personality/voice for my actual character. I also opted for doing all of my sneaking off screen. I usually have a conversation with my DM the following day to handle what I did, this usually involves any necessary rolls.

Additionally, if I want to do something secretly during a session, I text my DM who knows that I may text him with a secret action. He will usually quickly text back what roll he wants, I roll and relay my roll, then depending on the action he either says "you succeed/fail" or he will work it into the narrative. It is working out extremely well and is very fun for me, and also very fun for my DM who was actually hoping someone would want to play a double agent. It does require a TON of communication and also trust on the DM's part that you won't do something to horribly derail the campaign or get the party in trouble.

Work very closely with your DM

Any time you want to do secretive things (like being a secret serial killer) you need to work very closely with your DM and check before-hand to make sure your DM is ok with it. Keeping it off screen is a great way to handle it, and can even add a great element of surprise for the rest of your group when your actions begin to have consequences, but if you keep it from your DM then it didn't really happen.

DM's word is law.

Manage your expectations

While you think the idea is great, your DM might not really be into it. They might have reservations or not want to risk you messing up the campaign with a stupid action. If you try to do something that concerns them they might say "no". You will have to be prepared to be denied doing something you want to. Also, your off-screen actions will have on-screen consequences. Be fully prepared to be targeted, arrested, or even killed. At the end of the day, the campaign is for all the players and the DM, not just for you.

Lastly, some characters must end

With a secret character like this it is likely that you will be found out at some point. If this happens and nobody else at the table (DM included) seems to be enjoying playing with your character anymore, hang up your hat. Have the character die, run away, whatever, but don't sacrifice the fun of the party for a character that is likely to get the party killed or is taking away fun for the rest of the table.

That isn't to say immediately give it up....there are some great RP and character development opportunities hiding behind being caught at being a serial killer. If they discover it late enough, the party might even vouch for you or try to help you. I love some good RP driven character development. My point is at the end of the day you are all [hopefully] friends getting together to have fun. Pay attention to the mood of the party. If you are the only one having fun, you need to rethink your strategy.