First, evil can take many forms and can vary significantly between people, cultures, and time periods so you’re going to have to dial that in on your own based on your story.
Second, (most) people don’t wake up one day and say “Hey! I’m gonna be evil today!” There’s usually a reason and it’s up to you–yes, you–to make it believable. Some people are that petty. Period. You don’t have to look far to see people completely overreact to even a perceived slight. Disease, and not just mental illness, can cause violent changes in behavior, too. People who were once loving can transform into callous, vindictive monsters.
Here’s a couple of my golden rules for writing villains:
Hannibal Lector gives no fucks. Lector wasn’t worried about upsetting anyone while satueeing a victim’s frontal lobes. He wanted something and he went out and got it. Simple. He’s a particular type of villain, but to me he personifies it well.
Develop them! Without an antagonist or villain, you don’t have much of a story, so I’m not kidding when I say you need to develop them as much, if not more, than your hero/protagonist. I’ve got a few villains that make my skin crawl, but I still do the work on them.
Note: If you’re going to use a medical problem, such as a brain tumor, for a person’s behavior, do more research than you think is humanly possible and get a sensitivity reader for the love of all that is good in the world.