Someone who is malicious enjoys hurting or embarrassing others. If you're writing a book about good and evil, you'll want to come up with a truly malicious character to do all the bad stuff.
Malicious is the adjective based on the noun malice, which means the desire to harm others. Both words come from the Latin word malus, for bad. If someone is malicious he doesn't just make bad things happen; he loves to make bad things happen.