Bullying wasn't the sole cause but just one of the many triggers. Even them screaming out if there was any jocks as soon as they entered the library gives you some insight into their thought process after the bombs failed to go off.
But everything done that day was done strictly on impulse and adrenaline after knowing they completely failed with the bombs.
Even though they failed Eric Harris is probably still content with the fact there are still plenty of people referencing and copy catting what he and Dylan did, (wherever he and dylan are). Overall, I think that was Eric's main goal. His anger and hatred just fueled things but he wanted to be notorious through violence.