Prolly something like he started the fire or she helped him escape the fire but nico ended up getting caught in the fire because of that and he feels its his fault she died.
(〜￣△￣)〜yeah probaly,
He didn't kill her I mean deliberately at least, the fire started because of a short circuit which means there was probably something wrong with the installment of it or it was just old. He feels guilty and with good reason because he told her his brother was still in there so she should be the one to go in to look for him. Basically because he was too much of a coward to go in there himself, sadly he later realized that his little brother had already been rescued.
Wait, whattttt???? He kill Nico ?? Can someone explain me ?