How do you explain a person who knowingly continues a behavior that is hurtful to others?

If someone is emotionally abusive, and they know they are emotionally abusive, and say they don’t want to hurt others, yet they continue to be emotionally abusive, how would you explain that?

Are they mentally ill? Are they sadists? Is there something wrong with them? Are they just mean people? How do you explain it when someone continues to behave in a way they know is hurtful?