Homophobic ppl do crazy shit, he felt like he got stained so he had to ruin his friend too sneku-kun
Plus, the guy was very intimate with his brother at times, "brothers" don't do that. Maybe Minwoo is a closeted bi guy or guys are just very chummy with their friends?
I don't completely understand why Minwoo was so upset about the kiss. He could have just ghosted him after the accident.