The tax collectors.  But he is not calling them mentally ill.  He is calling them spiritually ill.

Big difference.  Often enough the mentally ill cannot control their compulsions.   Sinners can, we just dont WANT to.... 

 Debatable, but now at least I understand your point.👍

I would maintain that living in denial of the reality that tax collection is immoral (because it is theft) is clearly a form of insanity. 

 I guess some are in denial.
The others are simply ignorant.
Ignorance of life is the opposite of enlightenment.