I did some digging to see if this was a case where he could have just been having sex with his 17 year old partner, and:
"Arizona has what is called a “Romeo and Juliet law,” or close-in-age exemption, that is designed to protect underage couples from prosecution if they: Engage in consensual sex. Are at least 15 years old. Are less than two years apart in age." https://www.suzukilawoffices.com/blog/2016/october/what-arizona-law-says-about-statutory-rape/
Iirc he did 12 years for his crime. Now, that doesn't justify his murder - the shooter had no idea that this specific guy had done this specific thing. He was just killing protestors at random. But the dude was, at best, caught having sex with a fifteen year old.