Updated: Feb 26, 2020
Most villains, like heroes...are a result of circumstance. Something dramatic happens, which completely changes their way of being. Examples: Bruce Wayne's parents are senselessly murdered, and this fact eventually leads him to become the crime fighter famously known as the "Batman." He could have easily become a villain, and if it wouldn't have been for Alfred, he more than likely would have.
On the flip side of the coin, you have Oswald Cobblepot, aka "the Penguin." The Penguin was once a nice guy, who was teased very much in childhood due to his short stature, obesity, and bird-like nose, these traits made him an outcast in his rich, debutante family. Their rejection, drove him to become evil.
Once good guys, who were treated badly by hurt people, villain's conclude...what's the point. Their transformation begins by seeking vengeance on all those who mistreated them. After everyone (in their mind) has been justly dealt with, they conclude that others will eventually begin to mistreat them as well. So, instead of giving them the opportunity to do as they believe, they begin the process of elimination. Eventually, giving people what they feel they deserve becomes second nature, and creating new ways of doing it...exciting.
But seeing that who they use to be is buried deep inside their subconsciousness, something or someone tends to bring it back to the surface...if for nothing more than a brief moment. If the villain's a man, it's usually a woman (mother, wife, sister, etc.), and vice versa, if it's a woman.
None of us are born bad, this is a behavior that's taught by people who learned how to do it...quite well. The skill of being bad is mainly fueled by pain, pain that was experienced physically, as well as mentally. Villains are hurt and want others to literally feel their pain. Just my humble opinion.