This already happens, sort of. Mostly with vampires -- they usually have to flee from town to town, getting kicked out "after appearing not to age." But no one hunts them or anything, which should probably be a thing. Vampire kings don't get deposed because they "impose oppressive edicts" on their people, which I assume includes quelling resistance.
Since necromancers usually live in towers or camps it's less of a problem for them; but technically anyone with the [NO_AGING] tag is subject to suspicion, I think.
I dearly hope for vampire hunters. Right now DF lacks something that would definitely keep vampires in check in every universe. Vampire hunters. From Simon Belmont to Van Helsing, they all have been the ultimate terror to vampires. It remain to be seen, once is implemented, how guilds, groups, religious sects and orders would affect the world, but I have no doubt there wont be a lack of vampire hunters (that would also hunt other assorted monsters). That could be the crucial element of balance to keep vampires in check.
By having one of such guilds on your civilization or laying around in a neighboring one, you could expect regular visits if there is anything suspicious, random trials a la Spanish Inquisition (but they you can' expect ok?) or perhaps you can even call them for help with your liaison once you suspect something funny is happening in your fort. But this could go the other way around too, if you intend to make a vampire fortress then they become a hinderance, and in time they could be added to the list of sieges or even "excommunicate" you from your civ if they fail to clean the fortress either by action or inaction form your side, basically making everyone and their grandmother you enemy.