I see a lot of people talking about how stupid or cringy or impulsive he is to be doing this, but they seem to be forgetting he is basically a preteen/barely a teenager. I think he is like 12 or 13 here. Even if he has all his memories from his past life, he is still in the body of a teen; AKA a seething pot of hormones.
He saw his fiancee and girl he liked/loved, who came all the way from another country to be with him, getting shit on, treated like something dirty and unfit to be her, and likely about to be hit.
In addition this is a girl who one of the only people to treat him and his mother with kindness and respect when his own father slighted them.
She is also someone who is sweet enough to hear the shit they are saying about her, but still able to put on a brave face try to cheer him up/make sure he is having fun.
On top of all that he realizes that the situation parallels his and his mother's own in Hyme and dredges up all of that baggage.
Combine the above with the roiling hormones and lack of impulse control someone his age would have, speaking biologically since his body is that age despite his mind likely being older, and I am surprised he stayed as composed as he did. While they didn't really show it he was probably seething with rage and it is probably only thanks to his training that he didn't just attack the asshole. People seem to forget how young his body is and just how much hormones can influence your thoughts/actions.