Power- A villain must be at least as powerful as the hero, maybe even a little stronger, because the two forces need to be an even match. This doesn't necessarily mean both characters need super powers- they can be equals in influence, intellect, etc.
Intellect- A villain must possess a strong intellect. It must be believable that they have built the world of conflict for the story to take place. They must always be at least one step ahead of their competition and be prepared for the hero to try and stop them. A good villain is never taken by surprise.
Morality- A villain walks a finely drawn line of morality, teetering to one side or the other to suit their needs. A good villain is in control of their own moral compass and would show no hesitation if presented with "The Trolley Problem". They would simply weigh the outcomes and act in their own best interest.
Ambition- A villain needs to be ambitious enough that they always have a plan in the works. Most stories revolve around the hero's response to the actions of their villain. Without a villainous scheme, or plan, the events of the story will seem farfetched and unbelievable.
The actions of the story also need to be believable based on the character the villain. If you want, at some point in your story, for there to be a fatal fight between siblings then you must foreshadow that event. A flashback showing the villain as a child- taking the life of an animal or being obsessed with death- would be enough to set up a future event and make it believable.
A Redeemable Quality- Every villain needs to have a redeemable quality... because people want to feel that the villain doesn't always have to be the villain. Audiences usually like it when a villain is drawing their anger from a wounded place inside themselves. Giving a villain a redeemable quality (a loyalty to their family, a difficult childhood, etc.) is excellent for character development and making people care about the overall struggle.