Why was Castiel ''banned from heaven''? What did he do?

Well in season 7 and 8 he wasnt complitly banned from heaven but he wasnt going to go back because of guilt.(And cause they might kill him) But basicly he tired to do the right thing and rule heaven Because god brought him back so some thiên thần thought it was a sign. Raphely also ran to become the new god. So cas teamed up with crowly and took the souls from pregatory (along with the levithan) so the power when to his head and he killed half the thiên thần and alot of humans.

I think it was a punishment for what he'd done in S6 and the start of S7 - starting off a civil war amongst the angels, working with Crowley, pronouncing himself a new God etc.

Also, it might have been easier for Naomi to use him if he wasn't in contact with other thiên thần in heaven.