As long as we're placing bets I'll guess a very unconnected guy who genuinely dislikes those to whom he sent mail. He probably has right wing leanings, but not ultra right, and isn't a total moron IQ wise but is below average. I'd also guess that the motive was more mockery than fear and zero intent to do physical harm.
If I'm right or wrong, the crap he did wasn't funny. If he is left or right his actions ultimately speak more about himself than Trump, CNN, Biden, DeNiro or anyone else.