Zombie and necromancer citizens?

[Nautilusman77]

I have a fortress that has around 3 necromancers and a half a dozen "sallow zombie" citizens. During a scuffle with some giant eagles one of the necromancers freaked out and started to resurrect friends and foes alike in the area. Whats weird is that the purple zombie dwarfs that he resurrected seemed to be fighting alongside my other citizens. After dispatching the eagles they were immediately resurrected and posed a greater threat even taking down 4 of my military dwarfs. The necromancer died in this whole debacle and I was able to get things under control.
I'm a little confused as i thought undead were meant to always be hostile to the player. on top of this these "sallow zombie" citizens seem to be peaceful. Are these guys a danger to my society or can I just draft them into the military and have undead warriors of my own.
As a bit of additional info I am having a vampire situation with people popping up desanginated.

[AlP]

Recent updates introduced some complexities to the way undead work. Your sallow zombies are probably intelligent undead, and there are other types of creatures as well. You can read about them on the wiki, or just experience what you see without spoilers.

Same with vampires, there are ways to find out who they are.

[malthenielsen]

The undead with cool names like "sallow zombie" and such are intelligent undead. They have the same likes and dislikes as when they lived, so they're loyal to you.

They're physically strong and might have extra powers to them. Like some of them can fire off ice shards from their hands.

Some people even do tricks to get their dwarves to become intelligent undead. LIke drown them and then have the necromancer see them while releasong something to attack him so he resurrects whatever dead bodies are around.