Tyrant
A despot.
vicious person
harsh
One 4 letter word meaning a joke or cruel trick is hoax.
Psychopath.
uuuuuuuuuuuuuuuuuu
cruel
I'm going to assume that you made an error by starting this question with "why" instead of correctly using "what" but I'm completely fine with that. Anyways, to answer your question, hippipotomonstosesquipedaliophobia is the fear of long words. Whoever made the definition / word for this was a cruel, cruel person.
Psychopath.
A person who portrays life as harsh and cruel is a pessimist or a cynic.
Cruel as a cruell person
slap another person