To be fiendish is to be cruel or evil. You might choose not to watch horror films — or soap operas — if you're not a fan of seeing people do fiendish things to each other.
Fiendish behavior is terrible and vicious, and fiendish people behave without kindness or conscience. An informal, less serious way to use the adjective is to mean "complicated or difficult." You might, for example, describe your fiendish calculus exam or rave about the fiendish plot in your favorite book. Fiendish comes from fiend, "evil spirit," with its Old English root feogan, "to hate."