There is many reasons/ways human can become evil/sadistic/cruel/sick/etc... A more common way(well with dealing with abusers, murderers, rapists, etc..
Someone made a theory(and it commonly accepted) that all humans are evil in a sense they have sadistic thoughts, and ideas about hurting someone. Chances even you(maybe you felt like killing your ex..or..something more mundane like you know just kicking him.
The thing that stops people from actually acting on impulse is common sense, parental teaching of right and wrong, and self control. It is theorized that people who caused murders and crimes think differently and do not have self control.