I've finished reading but I still don't get what made Seth change so much after season 1? ...

What made him change was the fight. (At the end of season 1). The girl he loved was scared of him after the fight, and her dad told Seth if she was ever in any dangerous he would not let Seth see his daughter again. However he was forced to leave the country because of the the fight, and then forced his way back into the country. That it way he got into so much trouble with the cops after killing that dude.