At some point, the 2nd Amendment Absolutists need to acknowledge that there is a tradeoff between freedom and security.
They need to admit that you can't make it harder for people like Omar Mateen to get assault weapons without making it harder for everyone to get them. And if you aren't willing to make it harder for everyone to get them, you're leaving an opening for terror attacks like this to happen.
Stricter immigration laws would not have stopped this non-immigrant.
I'm not sure how "mental health" does anything unless you subject everyone to mandatory psychiatric evaluations at regular intervals.
You can't just say, "Well we'll just take the guns away from the Muslims so that the LAW-ABIDING CITIZENS can have all the arms they want."
There is always a trade off in a free society. Its great that the 2nd amendment keeps it that way.
While you pointed out that this guy was an "american born", I do feel that we haven't confronted extremist Islamic ideology strong enough.