Often the first thing that jumps to mind when we hear “villain” is murderer, kidnapper, terrorist, or crooked cop. Technically, though, a villain can be anyone who has the potential to do serious harm to your hero. That can mean the husband stealer or the slanderer too. How much your reader wants to see them fail and get their comeuppance all depends on you. Just remember that sometimes the best villains are the ones we least expect.