Mike is straight up evil to Katherine

Rewatching DH and at the end of episode 9 of season 6 Mike walks in to Katherines house and sees MJ sitting there after he had been looking for him. Katherine says she just wanted to talk to him about what they had, that it was real. Mike then tells her that what they had was never real and that he was thinking of Susan every time they were together. I was shocked. Like I get why he was mad but that was straight up mean whether it was true or not. Why even be with her at all then?? Such a jerk! Also not excusing Katherines behavior that season but damn