I know this is an unpopular opinion but I believe Marco is a narcissist hidden behind a nice boy facade. While he did go out of his way to help Elle out with the dance competition, he crossed the line multiple times.
He knew Elle had a boyfriend. That should've been reason enough for him to not pursue her. He didn't respect her boundaries that come with having a boyfriend, he made it known that he was into Elle which obviously would confuse any teenage girl already struggling with her own relationship. He took advantage of the fact that she was upset about her own relationship and chose her as a target that he could win over from Noah.
After almost ruining Elle's relationship by kissing her, when she wanted to go talk to Noah, all Marco could care about was himself and that Elle didn't want to pursue what they had together.
He wanted to start beef with Noah at the game when he had no place to do that. With the amount of information he had from Elle's relationship struggles, only a best friend who has known Elle should be confronting Noah. In my opinion, he crossed the line BIG TIME.
Elle literally friend-zoned him. She admitted to having some feelings but told him he's not the one and she loves Noah. For some reason, this still wasn't enough to get it to click in his head that she did not want to pursue anything with him.
Even after everything happened and they had graduated, Marco left the movie on a cliff hanger line saying after his friend told him she's not worth it and he replied with "Yes she is". At this point, he's just being plain selfish. Elle made it more than clear that she loves Noah and that he is the one. I understand that things change but with the information we have from both movies so far Marco has shown practically zero respect for Elle's wellbeing.