There isn't a 'first come first served' principle in love, that's why Katagiri wasn't in the wrong I think. I'm not going to comment on the rape scene as I was kind of disappointed in Katagiri because of that, but I kept in mind that people have done worse at the hands of desperation. Sawada has also said stuff that should never be said to someone, so he's not much better. At least Katagiri was honest with his feelings. ┑( ̄Д  ̄)┍
Saying the wrong thing and physical violence are not comparable. I think that no amount of desperation justifies physical violence on another.
Also, his love and his desperation are his problem and not someone else's responsibility. "I love you" is not an excuse for taking liberties with the other person and hurting them. Minami doesn't owe him anything.
You might have a different view. Let's just agree to disagree.
Katagiri got into a relationship with Minami knowing fully well that he is in love with someone else and just got dumped. He was basically luring in a person who was in a weak state of mind. Then he raped him violently for being in love with someone else when he knew it all along and even used it to his advantage. He should have been sued and thrown in jail. Why do people think he should have ended up with Minami?