friendly ghouls

[Pete]

Had a raid from some necromancers, who brought some undead humans and dwarves. I killed the necs and human undead, but the surviving dwarf ghouls are now considered 'friendly'? Is this a bug?

[Lamp]

I've had intelligent undead visitors before. They're probably just chill people without the necromancer. Heck, maybe they hated their necromancers?

[LastChime]

Not 100% on 50.xx but in 47.xx there were a few types that can spawn, the intelligent ones may end up petitioning to rejoin your fortress, the non-intelligent ones tend to just linger indefinitely and murder hostiles so it's a kind of a win.

[McFuzz]

When a necromancer raises anything, there's a 50% chance it will be a dumb undead and be hostile to all living entities, and a 50% chance to maintain the allegiances it had in life

My guess is that:
The undead that came to site were hostile, allied with the force that brought them to the site
You killed one of them
The necromancer re-raised them
The 50% chance for the corpse to maintain the allegiances it had in life, succeeded. The corpse is no longer hostile.