In my opinion, necromancy is a very misunderstood subject, but you can't really blame people for frowning on it when it's literally raising the dead. To most people it probably just seems morally wrong to reanimate someone or something's dead body. There's also the fact that it could be seen as "going against nature" in the sense that you're giving life of sorts to something that is dead, and in nature, something that is dead won't just magically come back to life without intervention.
From my understanding, simple necromancy is just giving life to the corpse, and not messing around with the original soul (unless soulgems are involved). The more complex necromancy is manipulating the souls themselves into being corporeal and following your orders.
However, I do not know exactly how they make the bodies function in a simple summoning, so it could be they are just throwing in a random soul, which isn't good, or they are throwing in a Daedra, which isn't too bad all things considering.
Pretty sure Necromancy is a fine School of Magic, if the people wielding wouldn't be Dunmer and wear black robes at sacrificial rituals...
Jokes aside, I think that Necromancy is a fine School of Magic, it just needs to be controlled a bit. Soul-Trapping someone, on the other hand, is quite revolting and unkind.
If you forget the fact that necromancy uses the dead as canon-fodder, it's basically like every other branch of magic. Destruction is an accepted field of magic, even admired, but both good and evil people use it. It all depends on how you use it. If you kill for the sake of practicing or such, then it's a dark practice
As a note, if you use dark soul gems, which is arguably an aspect of necromancy, then your delving into the evil aspect.