LOL I do think people defending Roger and Natasha might have their priorities a bit wrong.
Good old Roger is portrayed as a bully, who has been emotionally and physically abusing poor Sammy for years and years without being stopped by anyone and abused untold others as well including Katie. Especially the way he treated Katie already had MC pretty angry before he ever went to the party. Katie is someone he cares for and not just to get into her pants. Even if she is not his sister, due to their past there is that family dynamic between them and to him she has been seriously abused and hurt by that duo. So yes he was quite willing to get physical with Roger, also if follow game you know he might have had his own bullies in past.
Natasha is more or less portrayed in the same way as Roger and every bit as abusive and remember the whole tape thing was her idea to hurt and abuse Katie even more. MC was there just to help Katie out, protect her and help clear her name in first instance. Then Natasha goes out of her way with her plan to give him an option and incentive as well to go much further as he originally intended by turning the tables on Natasha. Have no doubt if he had really been a douchebag new boyfriend of Katie instead someone who cared for Katie, Katie would have been hurt badly at least emotionally at the end of that party. As far as I can see Roger and Natasha got hung by the rope they themselves provided and deserved everything they got in the end and provided MC with all the motivation he needed to go as far as he did, even the Sammy part.
Sorry if you think you should sympathize with bullies and not with their victims as MC does, but the kind of manipulation and abuse that people like Roger and Natasha like to dish out created likely way more emotional victims and suicidal people that might actually been helped a little in their anguish and help them move forward. Now if you think MC only did what he did to get into Katie's pants and not out of some true anger himself about the behavior of those two in past also to poor Sammy, yeah then it might seem weird behavior, if you see him as getting angry about people hurting people he cares about with no reason then his actions do make sense. Payback is a bitch just as is Karma, poor Roger and Natasha.
Admittedly though with that model he might have shown himself to sometimes be to easily triggered, but not by Roger and Natasha. My two cents.