How did Devon get from having Dylan not mind killing her, to having him become one of her friends?

By including Dylan in things with herself and Zack so he didn’t feel like the third wheel and by talking to Dylan like he automatically was her friend. Eventually, he let his guard down and he found himself beginning to trust and like her as a cool person, as his friend.  He was no longer feeling as threatened with her as he initially was that she was going to just take his best friend away and leave him behind (which is during the time that he wrote how he figuratively ‘wouldn’t mind killing’(her).