I'm in favor of gun control, but this is probably a kid who shouldn't have had a gun.
At Maine, he punched out a window over an issue with a girlfriend. That was several years ago, but it suggests an issue with impulse control and anger. If those were ongoing issues, he should have done a self-assessment and realized he shouldn't own a firearm. I get that he could have done the same thing with a knife or a bat or whatever, but the gun certainly didn't help the situation.
From reports, he shot her nine times, then almost immediately kissed her and apologized to her. His lack of control caused him to kill a woman he loved, and left a daughter he loved parentless.
I don't know if this is a case where the guy just had mental health issues, or if it was due to concussions, or PEDs. It's just tragic that if this guy had done some self-assessment, both he and his innocent victim could still be alive.