And how can you know that, if you don't talk to them and see what they mean? If they go "I have an idea for an evil pc I think-" and you cut them off immediately with a "I said no evil pcs. That is no evil pcs. If you don't like it, take a hike" then how can you possibly know if their idea is something you would actually consider an evil pc?
Same question. How can you know this if you don't have the discussion? And is the DM compromising? They are compromising on whether or not a player can consider their PC evil, but not on the behaviors.
When did I ever once state that I don't listen to my players? But someone has to make the final call on all sorts of things, as far as I'm concerned that's the DM no matter which side of the DM's screen I'm on.