Do they give us all someone to focus our hatred towards, thus relieving the need to be hateful to our loved ones? Without lunatics (and maniacs) we would we all be being vile to each other as we would have no other target for our hostility?
Most people don't seem to go around brimming full of hatred towards lunatics, unless you define 'lunatic' as 'a person who is a member of a rival group or faction'. I mean, what makes you more angry? Someone who does terrible things because their brain is broken, or a purportedly totally normal person who chooses to believe or do things you don't agree with?
I know of several people who go around brimming full of hatred toward lunatics. They don't all necessarily have broken brain - they just make lifestyle CHOICES that a non lunatic would find abhorrent.
Without lunatics (and maniacs) we would we all be being vile to each other as we would have no other target for our hostility?