I know you probably don't know this coz you've not been paying attention to the facts:
But your first sentence is worded incorrectly.
1. He crossed State lines, i.e Went from the town he lived in to the town he works in. I did this for years living in Jersey and working in New York. It's not unusual.
2 He armed himself AFTER crossing State lines. Not before. The way you wrote it is illegal. What he did, isn't. Big difference.
3. There is no evidence suggesting he opposes the protest. Frankly, the only evidence one way or the other, suggests he didn't. He agreed black lives matter and was willing to help anyone in need of medical... That's caught on Camera by the way. Prior to the shooting.
However, he was opposed to burning or looting the property or businesses of those who had nothing to do with the incident that started the protest/riot. No sane person should disagree with that.
4. Finally, he didn't go there to shoot people, but yes he knew that those who wanted to cause mayhem and burn other people's property would not take kindly to someone opposing their irrational acts. So he armed himself.
5. For the record, 1 of the guys ge shot had a gun, another smacked him with a Skateboard. The guy that was with the first attacker also had a gun and shot in the air while his friend was chasing the defendant. So, yes it made all the sense in the world to be armed. As lots of the rioters were.