The last straw is over anime. Yeah, how is this good character??? I would understand if the last straw was being called coward but anime??? Dunno how people still seeing MC in positive way at all. He is bad character and that flaw isn't funny or good. That kind of flaw is something that will never go from his character. Why? All authors did this and don't expect he grow from that flaw.
Characters that can never grew from their flaws and become their shitty traits instead is more worse than character with normal flaw.
It's weird that it's anime; he's 24-25 years old, supposedly with enough intelligence to explore the stars.
My only hypothesis is that when he returned to being a 9th-grader, his frontal (and prefrontal) cortecis devolved to being 14 years old.
That's really the only thing that even remotely makes sense... amd even then it's not a good reason for his actions.
If it had been, as you said, a personal insult? I could get behind that, maybe.
For me, I am a big lad, and I've always been cautious about hurting others; calling me personally insulting names can upset me, but it has to be something like casually dropping the N-bomb, or real antisemitism, or some other kind of thing that affects all of society for me to get riled up like that, so I know that behavior is possible...
But....
Anime?
Grab a couple gallons of clue-juice, ya dummy; it's a great medium, but it's still an argument over art appreciation!
Get a-hold of yourself, ya dingus!