Maliciousness is the trait of wanting to harm someone. A bully's maliciousness may cause him to push smaller kids off the swings and laugh. Not nice!
Thankfully, true maliciousness is rare — the bad guy in a book or movie is more likely to be motivated by maliciousness than people you meet in real life. Still, if someone acts in a hateful or vindictive way, appearing to enjoy watching others suffer pain or humiliation, that's maliciousness. We can trace this word back to the Latin malitiosus, "wicked," from the root malus, "bad."