Just wanted to comment on this briefly....
As someone whom has struggled with mental illness in the past (severe ptsd, to split the hairline here), I can attest that qualified psychiatrists have a sort of "danger scale," which they use to gauge the level of hostility in their clients. Take me, for instance - I will be the first to concede there was once a time when I had no business owning a gun, albeit the person I was probably the most dangerous to was myself.... I've since made a full recovery and been pronounced cured by my physicians - this has just happened within the last six months or so. Now, I personally don't like guns and don't own any - however, I won't stand in judgment of those whom struggle (say) with MINOR psychiatric difficulties and own guns as well. From my experience, most people whom experience mild issues are in no way a danger to anyone - however, I do hear what you are saying. I guess there are no guarantees - one thing I've learned is that nothing is 100% - the only thing us humans can't escape is death.... They told me that when I joined the Navy and it has taken me many years to finally grasp the meaning of that phrase.
I guess what I'm saying, NP is that you have a valid point. From what I understand, the Colorado theater shooter (Holmes) was in treatment with a qualified psychiatrist before the shooting - he basically pulled wool over her eyes. So absolutely, nothing is 100% and guaranteed.... Generally speaking, however, I would tend to agree with Goshin, as MOST people who struggle with minor disorders are completely harmless.... Furthermore (from my experience), those people tend to be better off than normal people who've never been treated for any mental disorder.... But maybe that is another issue.
Please know that I do hear what you are saying....
edited to add: Now, some people are diagnosed mentally ill these days whom are completely normal - but, that is another topic altogether (I could go into what I believe to be a lot of over-diagnoses by a lot of over-zealous psychiatrists these days, but now I am getting off me own topic)... Anyway, hope all of this clears up my intent for this thread...