Ahem. Hob's Fall Cave in Winterhold region. You go inside, and hear some woman desperately calling out for help. By the time you reach her, Necromancers have murdered her in some necromantic ritual to an entity who is understood to be Mannimarco.
Fellglow Keep. A group of mages are vampirizing passers by they capture and then experimenting on them and just using them as destruction magic practice targets.
Sild the Warlock in Rannveig's Fast has quite a little schmick bait trap going on to get him some fresh test subjects.
Black soul gems are also commonly found in Necromancer lairs. Typically those involve the killing of another mortal to make use of them. Now, which would a necromancer rather deal with... a traveler on the road, or a bandit that is a physical threat?
If a necro simply found study subjects, meh. But the typical necromancer seems to have this impulse to make test subjects.