Dwarves can become serial killers instead of going stark raving mad, melancholy or berserk.
Any visitor or migrant to your fort could already be a serial killer. Rather than just start random fights, Dwarvern serial killers pick an obsession ( race, hair color, anyone named Urist etc ), then quietly go looking for a target in the fort before disposing of them in a variety of fun ways;
1. Murdering the target in their sleep - DSKs will first seek out a murder weapon, then kill the target without an announcement.
* option; stash the bloody murder weapon in another dwarfs room to frame them when the captain of the guard starts investigating.
** cunning option; hide the bloodstained clothes ( and swipe another dwarfs clean socks ) to throw off the scent.
*** Fridge option; Hide the body in nearby containers so that it's only noticed when it stinks up the place.
2. Murders of convenience - Pushing other dwarves down wells, into magma, pits, spike traps, or just locking them in their rooms until they starve to death. Dwarves could also murder hospital patients, but this might be hard to spot given the normal state of dwarvern healthcare.
3. Poisoning food with various syndromes ( or Armok forbid, booze ). Preferably the targets favorite food. DSKs can either poison the raw ingredients ( and risk discovery ), or the food itself if preparing meals. As there's a fair chance the wrong dwarf will eat the poisoned food there's potential for much fun.
Once a murder is discovered, it's up to that drunk and lazy captain of the guard and the negligent chief medical dwarf to investigate and find the culprit ( or more likely a huge list of potential Urists without a good alibi ).
If not caught, DSK's will then go back to normal for a season or two before selecting another target, waiting for another migrant wave, or sneaking out of the fort to assume a new identity.