Mental Illness As A Scapegoat?

When a disgruntled postal worker thought that he was unfairly dismissed, he went back and shot his co-workers.  "Going postal" became the phrase for getting violent.  But in this case and many more, the shooter was acting out because he felt he was wronged.  He did not have any mental health issues. But, recently it seems every mass shootings were performed by an unstable, violent individual with mental illness.  But I have questions about this?  Why are all the shooters male?  Females suffer from mental disorders also.  Families say that the killer seemed depressed.  One of the main symptoms of depression is guilt.  Past mistakes can be agonized over for hours.  I can't see a person suffering from depression going out in public, look directly into the victims' eyes and commit cold blooded murder.  Then turn the gun on themselves.  Most likely scenario would be for the depressed to stay at home.  Quietly commit suicide with inconveniencing as little people as possible.  That leaves me with two conclusions.  The first is that we have the worst mental health system possible.  Families, friends, neighbors, co-workers or school official did not notice any odd behavior.  If they did think the future shooter was dangerous no one reported it.  Even with all the shootings happening no one thought there might be a threat.  Did all of these people let this person down because they turned a blind eye to him?  Did any of these killers have any recorded mental health issues?  If so, what wasn't anything done to help prevent these violent acts?  My other conclusion is with using mental illness as the cause for every shooting, this eases the conscience of the politicians so they do not have to open discussions about gun control.