First, read the timeline here of when Dylan just newly met Devon and about 1.5 months in, she begans to date his best friends, Zack.
Dylan then writes about Zach Heckler in his journal 7/23/97:
“My best friend ever: the friend who shared, experimented, laughed, took chances with & appreciated me more than any friend ever did has been ordained … “passed on” … in my book. Ever since (Devon) (who I wouldn’t mind killing) has loved him … that’s the only place he’s been with her …if any- one had any idea how sad I am … I mean we were the TEAM. When him & I were friends, well I finally found someone who was like me: who appreciated me & shared very common interests.”.
So, for one thing, at this point in time, he hardly knows Devon and to him, she is the woman that has stolen his bestie away.
Now, read here and here regarding his nasty ‘kill’ comment about Devon in his journal.
After this incident, Dylan still had nearly 1.5 years beyond getting used to Devon as an item with Zack and they ended up becoming good friends. Devon calls Dylan her best friend. Devon invited Dylan to her sixteenth b-day party in July ‘98 and they slow danced at prom in April ‘99. They even talked about going to see a Chemical Brother concert that summer. They were friends. In the early days, Dylan did not know her well save for the fact that she stole his best friend. Of course he wanted to kill her; he was sore about what she stole from him. If you were in his shoes, wouldn’t you be?