>>290880268
to take advantage of him

>>290880268
Shirou felt bad that Shinji had no friends.

>>290880268
Shinji was a lot less of an asshole when they were in the club together. In fact, most of their time together stopped after that. Shirou beating Shinji's ass and having Issei pull him away after he admitted to smacking Sakura helped.

>>290880268
Didn't Sakura claim that he was pretty nice at first? Maybe gramps mindfucked him.

>>290880310
It was realizing Sakura was meant to replace him as the Matou heir that set him off.

His arrogant attitude is a coping mechanism for losing the one thing he saw as what made him a person. Shirou being a mage, something he's actually known for a long ass time, grates at him. But it's also hinted to be one of the reasons why Shinji was ever friends with him in the first place

Because he's a nice guy

>>290880268
Why couldn't he just fuck his step-sister, be happy, and stop pursuing dangerous activity

>>290881415
because she wanted it, he didn't have super powers, and he couldn't win the only way he thought he could get them

The VN doesn't convey it well but he only became an asshole to Sherou recently.