"Get rid of guns, and less people die. It's simple really, to those of us in the rest of the world."

The word you are looking for is simplistic and does not apply to the US. Here in America our experience is that the more guns Americans own the fewer gun homicides and fatal accidents there are.
If you are mentally unstable does that mean you are too stupid to ask a gang member for a gun? Are you too stupid to buy gasoline or rent a Home Depot truck? We already forbid the insane from legal gun ownership.
If someone is a homicidal maniac we are supposed to pretend that somehow being denied a legal gun will keep people safe from them?
Really the solution is to not let known homicidal maniacs like Nicolas Cruz walk around free.

Are the terrorists taking pharmaceutical drugs that "may cause suicidal thoughts or actions"? And having everyone report to the FBI and local police about how crazy they are?

Funny how you want to blame an inanimate object instead of the killer or the products that say right on the label they CAUSE people to kill.

We have 1.5 million gang members in the US, you want to disarm the law abiding and let the gang members keep their guns?