seriously, im tired of correcting people that someone who is always sad is not a sadist, sadist can be defined as someone who derives pleasure from inflicting pain on someone else.
In other words, that girls thats always looking sad and never lightens up is not a saidist while that proffessor that finds happiness and bliss in seeing that a student spend 10 years reading a 4 year course, or that theres a mass failure.

and also you that enjoy beating those stupid pupils you teach.