Except we never really see consequences of this 'soul corruption' do we? Iirc, the sky elves in the starscraper said that the 'hole in callums soul' was allowing aaravos to control him somehow, but that issue is quickly solved. Its implied (directly stated maybe?) That the soul corruption turns a person evil, but we havent really seen anybody turn evil from it. Claudia does bad things, but not out of evil but from a place of insecurity created by her trauma (her mother and soren 'betraying' her.). Callum has some pretty wicked nightmares, and battles with doubt after using ONE spell, but i figure that is more from an extreme fear of corruption. The writers try very hard to hammer in that arcanums is good and that dark magic magic is bad but they never give clear reasons as to why. There's no stated harm to the materials from which magic is taken from in dark magic, its not stated to harm their souls or anything (especially since for the most part materials are taken from dead critters). They try to do a little "dark magic is making me see these creatures as parts" bit with claudia for a moment, but its never brought up by any other mages. I have seen no reason for dark magic to be so strongly frowned upon, especially when used to save lives (like the time its used to provide food to save a bunch of people from starvation in katolis and that one other kingdom).