I killed something in corpusium before I knew I shouldn't do that, now the dude won't even talk to me. Is there a mod or some other way to get cured?

He will help you again if you can raise his disposition towards you. But since his greeting ends with a 'goodbye' you can't bribe him. So the only way to do this is by using a charm spell on him and/or fortifying your own personality (drink many Telvanni Bug Musk).