1. Villains are clever and cerebral
Villains are the engines that drive suspense. They should be architects of the novel's inciting event and follow-up plot twists. They are insightful and clever. They excel at identifying their adversary's weaknesses and finding ways to exploit them.
Villains are devious. They should always be plotting and planning ways to catch the hero by surprise and make the hero suffer. If your villain is not laying "traps" and plotting pitfalls, then he/she is not doing their job.
3. Villains bring the rain
In other words, when things are going bad for your hero, a real villain doesn't let up. Villains kick heroes when they're down. They throw grease on grease fires. They show no mercy. They have no empathy.
Villains can never be trusted. Most people are trusting by nature and authors should leverage this when plotting. Villains exploit the trust of others to gain advantage and always look for opportunities to backstab both their adversaries and allies.
5. Villains eschew law and order
Villains are driven by self-interest and do not believe that society's laws apply to them. They behave in ways that are either immoral or amoral. Villains lie, cheat, steal, smear, blackmail, and malign. They don't play by the rules because rules are for suckers.
No way to sugar coat this one, villains are sadists. They take pleasure in the suffering of others. This suffering can be psychical, emotional, spiritual, psychological or, preferably a combination thereof.
7. Villains are willing to burn it all down
Metaphorically speaking, villains are arsonists. In order words, they will scorch Earth if that's what it takes to win. Even if it means sacrificing their own resources and allies. Villains are willing to "lose to win". Heroes are not.