It's a pattern that befalls most pre-fascist states, that pattern being swaths of young men become disillusioned with the state of their country. Most of them will seek out and side with the fascist side, hence the overwhelming majority of young men voting red. But sometimes that disillusionment can turn to things like this.

The dude lived out in Butler country, which is basically middle of nowhere PA. Pittsburgh is close, but a country like that just generally will only have shitty jobs, exacerbating whatever pre-existing feelings of existential doom the shooter had. So the shooter probably said "fuck it, my life isn't going anywhere, may as well do something useful".