"Mental illness" has become the catchall phrase for all deviants and criminals to hide behind. If you shoot up a school, you are "mentally ill." If you do drugs you are "mentally ill." If you are jobless and smoke pot all day, you are "mentally ill." If you are living in the streets, you are "mentally ill." Of course it is a perfect cover too: All mental health professionals are forbidden by law to say to press or anyone that they are treating such and such person. So you can do whatever you like, then when you face consequences, just use the magic words: "Mentally ill." Off you go.