Personally, I think the animation is alright. It's not the best by 2021 standards but let's keep in mind that this was 2004 and most people probably didn't have access to graphics tablets.
I see a lot of people here be over reactive and spaz out just because he was a murderer which I'm not trying to deny, he was a murderer and he commit a despicable act. However, when it comes to flash stuff like this, while it was alright animation wise, it also gives us an insight on what kind of person Jeff was. He was a depressed teenager with barely any friends and obviously saw art, flash animation and horrorcore music as his main positives in life.