1) How dared he indeed!! Giving it a second thought, though, Gedeon rashly planned Nathan's murder out of utter fear for Eva's life/safety. Gedeon hasn't got time enough to address things properly and make up a decent, detective-proof alibi for himself. As Detective Graves says, Gedeon needs Eva's reaction to be true, which means he has to be that cruel to her. Unfortunately, Gedeon's being "no saint" hasn't prevented him from coming up with a real botch-up.
2) She was bound to, poor thing!! Because Gedeon also needs Corinne's reactions to be true to invalidate his motive to kill Nathan in the police's eye as well as in the public one. But I'll bet there's no one out there who doesn't think she deserved it!!!