Possible motives:
1 Life long secret agent paid well,worked
for nation state to cause disharmony in US.
Was willing to die for the cause.
Could be one of many states. Follow the money.
2 Took 64 years of crap and was full to the brim
drugs will alter decision making.He held in his anger
and then exploded because he didn't find a good outlet.
He thought everyone was an asshole.
But not everyone is.
3 He recently joined a terrorist organization.
Due to 2 above.
4 He had a secret political motivation and believed
the ends justify the means.
But the ends reflect the means.
5 He watched to much news, refer to 2 above.
6 Something went wrong in his brain.
7 He was a life long criminal that didn't
care about almost any other people and never got caught.
Went out with a bang.