Mentally ill find a way to kill as we've seen numerous times with the use of knives and vehicles - poisonous gas, etc.
Again, I think this also is up there with blaming video games. You could claim that all mass shooters are by definition mentally ill - because they have done something "crazy" that clearly isn't "normal" or "sensible". But it doesn't get to the root cause, it just saves questioning things further (and stigmatizes those truly mentally ill, who are not a danger).
They may have mental issues, but how much is because of over prescription of mind altering drugs? How much is down to poor counselling or life advice (or complete lack of any social structure in their lives).
There's always a pattern - these little shits have always been psycho nutters for a long time, and they have escaped consequences for for previous bad behavior. It's rarely a real surprise at what they do.
The question is, how do you identify and deal with people like that sooner? Why do they sail through a government education system without a huge flag attached to them saying "shouldn't have a gun" or "keep an eye on this one"?
Anyone who thinks the solution is "background checks" needs to think about exactly what 'background' is being checked.
There are some "no-brainer" checks that you would imagine would automatically disqualify people from buying guns - being on a terrorist no-fly list for example, but all the dems voted against that one ...