Ooooh, yes, yes, we're getting there for sure. You're aaaalmost to the realization. You've recognized that other characters who aren't the champion can be corrupt and/or evil! But we took two steps back at the end there, once again putting corruption and evil into the same category for the player. And Tollus being human still (a) may not be true, he could be a demon disguising himself as something else, which is something we've seen other evil characters do already, or (b) he could just be...refraining from sex with demons and thus not becoming physically corrupted and changed into a demon. The realization you very nearly reached is that evil can lead to corruption, and corruption can lead to evil, buuuut...corruption in this setting is a physical occurrence that is largely sexual in lore, and doesn't have to be evil. The player can undergo corruption, but cannot be evil. Tollus is a great example of a definite evil that may or may not be "corrupt" in the lore of the game. In a sense, he is equal and opposite the champion.
I think your problem is that you have no idea what corruption is, but by definition, you want the ability to be evil. Sure, fine. If that's your flavor, go for it. But you can't complain that every game ever doesn't allow you to be evil. It's unrealistic and childish. Games, regardless of genre, are a form of art and a medium for telling a story. No one is obliged to make a story that caters specifically to any one group of people. It is a choice. This dev team have largely chosen to not make one catering to murder-happy folks like yourself.