Robinson's motivation has been disclosed: he did not like his target, consider him "hateful," and ended his life because he claimed Kirk's "hatred" could not be negotiated; hence, assassination was appropriate.
We can all come to our conclusions about what that says about his politics, but please reconsider your claim that he came from a "perfect MAGA family." Robinson was a high achiever who was falling through the cracks in society. He won a scholarship for all fours years of college, and only completed a year. He was undergoing vocational training as a backup, but it isn't clear that he had any enthusiasm for that career. His father and he argued. Over his choice of partner on at least one occasion.
There are more guns in thus country than people. It's a bit of a straw man to say that "more guns" would have prevented this, and (save the ultimate irony that Kirk took this position), I see the argument that we should tolerate speech to the point we don't commit murder as the prevailing remedy on the right. Lee Harvey Oswald murdered JFK with a bolt action rifle (hitting a moving target more than once). Oswald learned how to shoot in the Marines, sure, but Robinson was intimately familiar with how to use that gun, which was a gift from his grandfather. Also...Robinson missed. No sniper worth their salt aims at the neck. Kirk was fundamentally unlucky, as the shot almost certainly was off-target and ended up being fatal anyway.
I don't think there is any way Robinson would have gotten away with this. It is true that his parents urged him to turn himself in (he surrendered after meeting with a family friend who was in the clergy and also former law enforcement). They had the weapon, they had his DNA, and they had friends who were at first skeptical, then shocked. Robinson was fundamentally sloppy. It was only a matter of time, but, I guess since things worked out as they did, that's just speculation.