I believe the motive is that he was excessively bullied in school probably to the point of insanity, and

[Uncle Joe]

if there is one thing that *rump represents in general and particularly in the modern day GOP is being a bully.

I would bet a dollar against a donut, that's what drove him.

[no_hypocrisy]

1. And

it isn't known yet if he was bullied at home.

Between being a victim of bullies at home and school would set off almost anyone. It would be non-ending.

[MaryMagdaline]

2. His motive is that he wanted to shoot his gun and inflict death. The ultimate power

trip. People around him did not detect anger because it wasn’t anger. It is 100 per inability to feel empathy. He couldn’t wait to hit a big target. Excitement more than anger.

Only empathetic people struggle to find a motive for this barbarity. He’s not like you. He never was.

[Uncle Joe]

3. Apparently some of his fellow class mates didn't feel empathy either

[myohmy2]

4. hard...

...to find a bigger bully than trump...

...bullies...

[maxsolomon]

5. Maybe. Maybe not.

Others have said he wasn't bullied much if at all - just a quiet awkward kid who took the Cons side in Debate Class.

And he was 2 years out of high school, attending community college and holding down a job.