They will still raise the dead, and I've heard that even undead necromancers retain the powers of necromancy, though I haven't tried to raise one yet. They shouldn't turn on you. As a member of the undead, who do not have factions and are merely friendly to all other undead/necromancers. You can probably get away with mutilating them yourself.
I'm not sure whether Toady thinks of this as an exploit, but I guarantee he will eventually fix the undead's faction-less state such that they can turn on one-another.
I'm not sure what makes some necromancer/secret-related groups friendly and others hostile, but their zombies are always friendly to any other undead, and the necromancers themselves are usually also friendly to vampire adventurers. I play a very modded game, and my "spellcrafts" mod specifically creates a bunch of new forms of secret-based tower-building classes of wizardry. Once, I set out to acquire them all, and at some point they all simply turned hostile to me. That could be because I got a quest to kill one of them, though. She wasn't happy about it, for sure, and because I had to back out and fast-travel away, she was gone when I returned. Never finished that quest.
Also, don't bother starting multiple threads. You could have asked these questions in your own thread or searched for similar threads. It comes up every once in a while.