The only problem is it is unconstitutional and will have not have any actual effect on terrorists getting guns. And Mateen could probably have killed a lot more people with a couple of gas cans on the front of his car and ramming the front door, anyway.
From what I have heard so far I think your idea would be too impersonal. I think he had to look his victims in the eye and enjoy how they were afraid of him now. There is also the question of whether he was struggling with his own sexuality since he had been to that club before. He killed so many who were Hispanic and somewhat “brown” like himself. Was it self hatred operating?