I’ll be the one to say that I’m not 100% on Fugo’s backstory.
On one hand, they added more details to a rather ambiguous plot of Fugo that was to be later elucidated on his own spin-off novel but I feel like that said ambiguity in Fugo’s past only added to emphasize his unstable temper. Back in the original publications, the tidbit of Fugo’s past that was of interest was that he beat his instructors with a heavy book during his school years which caused him to be ostracized which led to where he is now. The anime however added a touch of pedophilia to the instructor Fugo straightened up so it makes his anger justified, although still reprehensible by law. I don’t quite agree with this sort of characterization since Fugo was completely in the right to beat up his instructor while the off-shoot novel made it so that the instructor made fun of his grandmother who he respected which triggered the beat down. I definitely prefer the violent outburst that way with an insult taken too far instead of Fugo directly trying to defend himself since that drives the point home on his inner violence compared to the justified decision Fugo got to doing while simultaneously being accosted for it.