Humans are a murderous lot. With just four Humans on the planet, back just after Adam & Company strolled out of Eden, the ultimate vacation-gone-too-long, already the planet was too crowded. So the first post-Eden crime was a fratricide, and that has set the tone for Human interactions ever since. Poor Abel probably never saw it coming.
Of course, Humans have a history of blaming the consequences of their actions on others1) and for this reason they invented 'evil,' defining it as a vague outside force that causes their plans and 'good intentions'2) to fail. Evil is often confused with mischief, chaos, or the grander acts of mayhem, but it can be distinguished from these by the way participants in the acts either invoke an outside force3) or claim to have internalized that outside force as a source of authority.4)
Note: As a Human moral construct, the concept of 'evil' does not mean much to Kittymorphs, Robots, or Jokers. It's not that they cannot grasp the concept5) - it's just that they're too busy causing mayhem to care.