Given the world that Strickland finds himself working in, it’s not super challenging to discover why it is that he is so crazy angry at the world.
This is certainly something that he does on a regular basis, though in the movie he does wind up taking his rage to the next level when he gets caught in a complicated situation with one of his co-workers, played by Charlie Day.
When we first meet Strickland in Fist Fight, it becomes clear that he learned many years ago that one of the only effective ways to get through to his students is to completely scare the crap out of them.
Being a teacher, he has what should be one of the most rewarding careers on the planet, but instead he is surrounded by a failing administration and indolent teenagers who don’t want to do any work.