Isagi on field behavior is actually common

A lot of people say how Isagi becomes aggressive when on field, as if he is bipolar, but his behavior on field is actually a normal thing on football and you can see a lot of players who are good guys becoming aggressive on field due to the competitive environment.

Even Messi, Kaka and Rodri, players known by their good behavior went to the point of physically hurting their opponents when they got angry at the field.

So no, Isagi doesn't have double personality or is a kind of hidden psycho, it's just a normal behavior when players are in a way too competitive situation, and Blue Lock is a very competitive environment, so is normal for him become more aggressive on field