I think Jackaroodave makes sense.
The people I've heard about doing the shootings weren't mentally ill. They weren't depressed, or bi-polar, or schizophrenic, or anything we're used to thinking of as mentally ill.
They might very well be psychopaths, or regular folks who just "snapped". I don't know enough to be aware of the criteria, but there are undoubtedly experts who would know what to look for.

And average citizens shouldn't have assault weapons, period.