You just answered your own question my friend, that is just it, soul manipulation, by killing the host and forcing the soul back into it you can then control it and its powers or those it retains from life.
I don't want to sound snippy, but I don't think I asked something.
No need to add another skill, we already have conjuration but maybe when we reach a certain level we can decide which path we could take, an option for necromancy and normal-legal conjuration that has just as many benifits.
Necromancy is more like Enchant + Mysticism. Conjuration should return to being the skill Daedric, and only apply to daedra. I think we've all agreed that it is a joke how people summon zombies from Oblivion, or summon bears to help with the same skill as summoning Clannfears.
No matter what, I feel it is as different from conjuration as alchemy is different from conjuration, and should be treated as such, even if TES lore dont cover it like they should, other fictional books do and they are where alot of the ideas come from originally anyway.
True, they need to clean up their magic classes. Mysticism is the spell trashcan, with one of everything in there. The soul spells need to be split between Necromancy and Thaumaturgy, the telekinesis spell needs to be combined with some of Alteration to make the skill Dynamism.
I don't know that it should be a separate skill. There are assassins, they have their own organizations, but there is no "assassination" skill, it's more of a character archetype. Necromancers should focus on conjuration (undead should be animated rather than summoned, however), mysticism (soul trap, detect life, etc.), and the non-elemental side of destruction magic.
I'm hoping there is only an elemental side of destruction magic, and they play it up with the effects. Conjuration needs to get a divorce from Necromancy, it's a complete joke the way it is. It's a gameplay mechnism, not lore. We need a lore solid and relevant application of necromancy, not this slop they're shoving at us like we're going to accept it.