Dwarves can now hold grudges againts eachother. This is represented as "hated enemy" in the relationships screen. A dwarf will hate dwarves if they know that they killed a member of their family or one of their friends. This will extend to the murderer's close family if the grudge holder has certain character traits.
If a dwarf gets the "start fist fight" job, they will prioritize hated enemies. If a dwarf sees their hated enemy fighting, they will immediatelly attack them. If a dwarf sees a friend or a family member fighting someone who is not at least an aquintance to them, they will join and attack any dwarf fighting that friend or family member.
With certain character traits, a dwarf may assasinate a hated enemy by taking a weapon and attacking them with it once they find themselves in a room that has no other dwarves in it.
If a dwarf has a lot of hated enemies, they will also prioritize them during tantrums. Having hated enemies increases stress.
A dwarf hating their spouse causes a divorce.
A dwarf catching their spouse cheating(for dwarves, this is a depravity sphere alligned biome exclusive feature) will start to consider both of the dwarves caught to be their hated enemies and get the start fist fight job.
TL;DR Slaves to Armok: God of Blood: Chapter II: Dwarf Fortress: Histories of murder, hammerings, and murdering the hammerer