Diabolical means "evil." This is a strong word. Too much math homework might seem unfair, but it probably isn't diabolical.
Diabolical is related to the Spanish word diablo, which means "devil." In many religions and mythologies, you can't get any more evil than a devil, so diabolical is a powerful term, stronger than even despicable, which is another strong word for things that are hateful. Diabolical things are also intentional. If you got hit by lightning, that's awful and harmful, but it's not diabolical. If someone deliberately runs you over with a car, that's much more diabolical.