Ripping the soul out of someone, isn't exactly a nice thing to do. Many law abiding people in the world of skyrim correlate messing with souls > evil, necromancy isn't legal and is only seemingly practiced by people with no morals trying to achieve personal power regardless of who it affects.
But is using something that's considered evil against someone considered bad (such as some mages, bandits, thieves etc) necessarily a bad thing?
Would you judge someone negatively using something 'bad' against someone 'bad', for the greater good?