I still strongly dislike marijuana; I've known way too many potheads in high school and came across quite a few at work, and it makes you look like you're retarded. I legit thought one customer had a mental disorder until his friend told me he was high. u_u
That is bad logic. It is like girls who say, "I'm tired of being played! Guys are all the same!" or guys who say, "All girls are ****s!" No. Those statements just aren't true. They were developed in response to the interaction between that individual and examples of the demographic who meet their stereotype. Yet, I know not of a single person who rationally holds these beliefs... they realize that they were just subject to a few bad apples.
Also.. the "mental disorder" claim is most likely not true, either. They probably in all actuality exhibited few traits of any actual disorder. It is just the natural human reaction to chalk up behavior to something so easily explainable. What mental disorder did you believe him to have? What symptoms did he exhibit? I do not think you will find such questions as easy to answer as making the conclusion that you did.