South Park Headcanons

Kyles Aggressive Outbreaks and Guilt

Kyle has to deal with a lot problems of many different kinds - he aspires to be a straight A-student, he wants to be a professional basketball player but fails due to his physics, he's constantly ridiculed and attacked by Cartman for everything, his strict mother fights off anything even remotely obscene in media he likes; which falls back on him, he's always scared of becoming the people he doesn't want to be like and in turn transforms into these people even quicker, his relationships always fail, he is often blamed for stuff he had no influence on...

Over the time all of this culminated in Kyle just deciding to not be the nice, likeable boy anymore when he's around his peers. He gets annoyed or even angry very easily and threatens people that piss him off with beatings.
He sometimes even lets his anger out on Stan until he realizes he's hurting his best friend, upon which he abruptly stops and feels guilty.
But since he is not able or even willing to change back, he manages to scare Stan more and more until their friendship starts resembling the dynamics of a bully and his victim.
This gnaws at Kyle because he doesn't want Stan to feel unsafe around him, leading him to start becoming protective and possessive of Stan.