Also, murderers are already in the game, but dwarven personality and ethics makes them rare. If you mod dwarves to be more like goblins (higher cruelty, less empathy, less of a punishment for murder), you'll see a lot more murders.
Wait really? Like, in fortress mode? Just randomly? What do you mean?
Every race has a range of personality traits which they fall in commonly. It's a 0-100 distribution scale as I recall in the RAWs, with 0 being not at all and 100 being always, with a scaling deviation range, which I think defaults to 20 for most traits. Might be misremembering how it works though.
Dwarven society is naturally inclined to be loyal, hard working, value law and order and generally not be in favour of casual murder. Goblin society on the other hand is designed to encourage random acts of violence with empathy being rare, laws nonexistant, sadism common.
If you check the Legends you'll find a lot of goblins murder each other during worldgen. If you slanted the dwarves default mentality closer to the goblins on average there'd be a lot more random violence in the fort and it would probably be more severe, leading to strings of murders by fort members.
That's not how personality traits work at all. They're all 0 to 100 with 50 as the average by default. It can be changed to be between 50 and 100 with an average of 75, or even between 25 and 50 with an average of 25. There's no, not at all or always thing though. You say it as if there's a chance of having a trait entirely or not at all, when it's just sliding how much of a trait they have. Like altruism, which goblins don't have much of.
Anyway, it makes more sense that it's just related to world generation, I've seen demons just murder goblins with no explanation.