I am guessing he's angry because Geto makes him kill Itadori, that he believed is his brother and he discover that Geto is actually NaritoshiKamo from the past and is able to be in diff bodies so he tought it is also possible for his brother's for he feels a strong connection with Itadori the same way with his brothers the reason is still vague(I think its because itadori felt compassion to those 2 little brothers that they did not hold grudge to him when they died that created the bond?)