After the fact that he was so angry and always bullied... he bragged about a gun he owned... he left hints that he wouldn't be alive much longer, suggested mass murders as heroes, leaving hints that he would kill...
In a blind rage he killed the teachers, chances are they didn't bully him, but they added the fuel behind Toby's back to the students that did bully him... meaning what was said to the bullies gave them more reason bully him.... or the blind rage, he was taking zoloft at the time of the murder (I've taken that pill, and all I did was cry on it, back in 2009, now I don't take anything or bother with pills or therapy, it's for the weak minded, antidepressant, my *ss, it's a depressant lol anyways)... his blind rage, he just shoots the teachers instead, with a mind set of first person in this classroom, I see, I will shoot no matter who they are...