I get that he got traumatized by being betrayed, but like, we saw her interacting with him in the past, before everything went down. She literally called him hopeless then, to his face. He knows what she's like, he knows she's just a tsundere annoying sister. This is just him being a moron.
Heck, the guy who told him she called him hopeless/good for nothing is literally the same guy who said "she said you were hopeless and imcompetent, I hate trash like you, I'll work you like a slave until you die", shouldn't you take it with a grain of salt? But no, instead he perfectly amicably made up with that guy. Aint it weird how the one person who he believes slighted him over misunderstandings, who he actually gave an ear to and made up with, was a guy? Surely there isn't a pattern from the author here.