I've had
All visiting necros in my fortress were villains. Not only that, but one of them proceeded to slaughter everyone when I tried to exterminate a were visitor (helped by raising an ogre prisoner I'd just executed). When I exterminated the necro before tacking the were (before it turned, and after save scumming) I was "rewarded" with a loyalty cascade. Since then I just used DFHack "exterminate him" on all of those bugged villains and questers out after artifacts in my fortress (a lot of extermination, as the villains outnumbered the legitimate visitors).
We've only had one necromancer villain so far and his confessions yielded a lot useful information. He eventually went crazy and got himself killed. The faint ghouls have been incredibly helpful against the weres who are by far the biggest threat to the fort. One of them did turn out to be a villain, he's sitting in a cage indefinitely. Villains are super common generally but I haven't noticed that all night creatures all villains.
The only loyalty issue we've run into was a non-night creature Prophet who somehow got himself flagged as an enemy of the fort and caused panic in the temple level until we put him down. Didn't cause a cascade but I have no idea why it happened.
Recently though, the lack of positive thoughts have caused stresses and now has had a tantrum (nothing raised thankfully) and a depressive state while chained up, all the while going through reliving the death of another dwarf that slid into depression and had stopped eating. For some reason, the Necrodwarf keeps complaining about family and friends but doesn't have any family (and as such can't be exiled) and won't socialize. I'm thinking of tossing her into the bridged up section of my fort that has a vampire manager and see if doing nothing but staying in a generic temple branch, military training and doing nothing else helps.
This just seems to be an issue generally. It's slowly driving all our dwarves insane. Not having friends and family doesn't prevent them from having the need. I remembered Kruggsmash's "autumn festival" from Honeystoker and set up an R&R burrow restricting them to the hospital, temple and tavern areas. After a day or two they start to socialize and do other things they tend to normally ignore to the detriment of their needs. If you leave it too long they get ansy to work again and they only increase relationships a bit, so I don't know if it will save them in the long run but it does generate more good thoughts and take care of needs detracting from their focus.
As far as chaining them up, I've had mixed results. For tantrumy dwarves it's safest but if they don't snap out of their funk they can be locked up indefinitely. I filled the cells with different art so they get a lot of good thoughts when locked up and it seems to help. ("Well, I'm locked up, but that sure is a well made rope") But now I've only gotten beatings assigned recently.