Think thig is; are YOU becoming evil?
Like, the tadpole devours your brain and replaces it. Even if it is one of those rare Mindflayers that gain 'Partialism' (IE: Keep some of the hosts memories), it's still not YOU.
You are dead. Your soul is in whatever afterlife you have in the Forgotten Realms. Your brain has been devoured and you are no more. You have ceased to be. You have shuffled off this mortal coil, as Monty Python would say.
Unless something is done that renders the parasite inert, and you drawing on it's power allows it to take you over.
Kind of like relying on the Dark God Gig's powers in Soul Nomad & The World Eaters. (Deep cut!) You can use his powers to pwn in combat, but every time you do, he comes closer and closer to devouring your soul. And he's up front about that, too. He flat out tells you he will take over your body and devour your soul.
All. The. Time.
..Well, when he's not insulting you for being a weakling. :P
