There are so many unanswered questions here. It seems to me if this kid was part of a larger group, it would be to their advantage to make it look like he committed suicide. Why have all these other explosives in the apartment if there were no other plans to use them? No way did his roommate not notice them. And who would have access to the kid's computer? The roommate of course! Why couldn't he have left the message to get the heat of of him?
The blinking cursor does make it seem like he was interrupted before he finished the message.
Like, they've come to get me! Or, I gotta go now, the timer is running down!
The bombpack was obviously not a last-minute "throw it in the bookbag and worry about it later" kind of thing.
"While writing in his diary, Joel had a spur-of-the-moment thought to go out and strap on a bomb by the stadium."