In my lime of work caring for mentally ill and retarded I encountered psychiatrists and psychologists and I would not trust any of them to diagnose illness. That is to say there has to be more proof than the word of one of these individuals who could claim mental illness if someone is depressed.
I like to use this example. We could line five of these "professionals" up and get five different answers. Then we have someone deciding to define mental illness and removing guns from folks. We are seeing senior citizens under fire because they may be considered not able to handle their affairs. Have any seen stories of some senior citizen going on killing sprees? Other excuses have been made that a relative who may be "unstable" could use weapons so lets remove those guns. How about taking guns when someone dies so that the relatives can not have them?