Almost hard to believe this guy wanted to kill her at one point
It's not like he actually did- he was just experiencing some of the first inklings of puberty and decided to interpret the new impulses in a morbid way due to his existing interests. That time when they first went to the infirmary together kind of highlighted this fact when he talked about 'bugs' or a 'parasite' in his brain making him think 'weird' things, and then talked about them/it moving down to his lower body at one point.
It's all about puberty and the hormonal chaos it causes. The basis of Chuunibyou, or the '8th grader syndrome', really, which is a major focus of this entire manga, alongside the romance. Growing pains, maturing, etc...