Rationale for this role: Death messages give too much information, too reliably, allowing people to easily identify which killers are active. I feel that there should be an NK capable of creating false death messages (that is, making it look like another role did the killing), so there's at least one other plausible explanation for death-message-based reasoning. This is particularly an issue with Vigilantes, who can confirm too easily via their kill method, but it affects lots of other aspects of the game as well.
The Copycat Killer is a Neutral Killing role with two buttons at night. The first one chooses someone, living or dead, who you want your attack's death message tonight to spoof; the second one chooses who to kill (a Basic attack.) You only copy the death message, not anything else. The quick summary:
Role Name: Copycat Killer
Role Alignment: Neutral Killing.
Abilities: Kill someone each night, making it look like they were killed by someone else's role.
Attributes: Basic Attack, Basic Defense.
Goal: Kill everyone who would oppose you.
Other notes:
If you choose a non-killer with your first button - that is, a role with no killing capability at all - you get the "default" kill message for their alignment: Vigilante for Town, Mafioso for Mafia, Potion Master for Coven, and Serial Killer for neutrals. Vampires, of course, always produce vampire-bite kill messages.
The role does not otherwise consider whether a particular kill is believable (so eg. if you choose Vampire and target a town member, it will look like that town member was bitten by a vampire and killed - though that may make people think they were a Disguiser.) If you choose an Arsonist, your attack will seem to be your victim burning to death (which may be suspicious given that it's just one person.) If you choose Werewolf on a non-full-moon night, you'll produce an impossible werewolf attack that will probably immediately give away your presence. If you choose a Bodyguard, your target will seem to have been killed by a Bodyguard, which will often be blatantly impossible. If you want to hide your presence, it's up to you to figure out who people are and make your kills believable.
You cannot, however, fake Jester kills - they show the default neutral SK kill instead. (This is because there is no situation where a fake Jester kill would be believable, and because automatically alerting the entire town to the presence of a Jester would punish them too severely.)
In case it needs to be said, you are not actually copying any properties or abilities from your first target, just the kill message. Copying a werewolf's kill message, for instance, doesn't make your attack Rampage and doesn't make your attack Powerful - you still just deliver a normal Basic attack.
Your falsification is fooled by anything that would fool an Investigator. That is to say that if you copy someone who was framed, hexed, or doused, you would attack as a Mafia, Hex Master, or Arsonist.
You can use your killing to investigate people's roles, of course (by faking an unknown person and seeing what comes up in the morning.) This is a key part of the role. Some people might object that that's too strong; but I feel that solo roles are generally underpowered, and that "well-informed" solo roles are interesting. Note that it's not as simple as being a consig, of course - you can only thoroughly identify killing roles, and the fact that everyone sees your results can make it dangerous to try and use it to fake being an investigator (although you can still try.) You also miss out on your pseudo-investigation powers any time you hit immunity or otherwise fail to kill for any reason, and if anyone else attacks your target then your results can sometimes be muddled.
Some people might also observe that it's difficult to hide this role's presence with strict role lists like Ranked. That's true. However, even if they know a Copycat Killer is active, you can still conceal who you targeted, which will interfere with things like Vigilantes confirming themselves. You have more options to claim in general - you can try and fake Vigilante, or you can use your pseudo-investigative abilities to try and fake an investigative role.
Speaking of which, some people might observe that this role is (generally) more powerful than the current Serial Killer. By my understanding, Serial Killers (and all existing NKs) are slated to be buffed - they're underpowered. Being more powerful than their current state is a feature and not a bug.
Part of the idea here is that any easy, guaranteed public message - like death messages - should always have at least two possible explanations, making it harder for the town to immediately conclude key facts and introducing room for evils to lie or spin alternative theories. A bodyguard kill message with no dead bodyguard, say, could be a bodyguard who was healed, or one used by a necromancer, or a Copycat Killer faking a Bodyguard attack. A Vigilante killing a town with no following suicide could be the result of the vigilante being healed, or a Copycat Killer, and so on. Making confirming Vigilantes more difficult is one major benefit of this role, but not the only one; I feel like it could be used by clever players to deceive the town in interesting ways by manipulating their understanding of what roles are present or who's killing who.
An alternative version of this role would just choose the kill message from a list. This would remove the pseudo-investigative aspect, but would make the deception aspect more powerful.