Antagonist help please!

I'm trying to develop my antagonist for a story I'm writing, and he just seems really 2D. Basically, the reason he's the antagonist is because he gets brainwashed, then murders a bunch of people. I really want him to be a creepy antagonist that will scare the reader (especially if he was nice before the brainwash), but idk what his mannerisms/way of speaking/etc should be.

Any help would be much appreciated! :D

To make a nice character with brainwash in my opinion you have to ask yourself these questions:

1. How is he brainwashed. That can influence the behavior quit a bit and show what he was before. Is he tortured while brainwashing or caused the torture some kind of brainwashing? Is the brainwashing go unnoticed by the antagonist cause he watched or eat something, etc. then you think of what he was like while the brainwashing.. Did he eat something that brainwashed him-> He can be impulsive or just don't careful. Was he tortured? how did he endure the Torture, was He tough, Did he cry all the time, was he afraid, etc.
2. Who and why did someone brainwash him? is he famous or rich or has he special and or important skilles what makes him the reason.
3. What did the brainwash do to him? Did he forget everyone but he is still not dangerous he just forgot. Or did they force him into something he isn't like a serial killer with no emotions (<- this is your kind I think) their are different trics to make him intressting in that aria. Let him have little pieces of memories, a habit like nail biting or crack his knuckles that the brainwash didn't get out of him. Let him have an outburt of emotions he doesn't have a name for because something like a memorie or the past kill or event or something he sees that triggered him. Let him have guilt of the emotinol outburst because he has no idea of it is allowed or not. Of it felt good or just toned weakness. Let him have conflict. I hope that helps, I just read a lot of fanfiction in that matter XD and some of it are my ideas to improve it.

Watch this video for help on brainwashing, it's awesome: https://www.youtube.com/watch?v=H_7BHQdyrzg

As for making him scary, I'd suggest having him be brainwashed slowly. Make him be a good person who is slowly but surely corrupted.

To make a truly unnerving villain, the reader has to understand their actions. The villain is a person, not a plot device. They have their own motives and thoughts and feelings. The reader has to be able to put themselves in the villain's shoes and say "oh I understand why they did what they did". This is why they are scary, because a good person and can easily become a monster under the right circumstances. I don't know much about brainwashing so I can't help there, but I'm working on finalizing my villain right now so I understand your pain.

So, basically what happens in my story is that the protagonist becomes part of an experimental military group, seeing if magical ability makes soldiers better. However, since the leaders of the experiment don't really trust the group to obey orders, they start to experiment with brainwashing. The brainwashing itself is kinda like Bucky Barnes/Winter Soldier, with specific words triggering the brainwash.

The scientists lock all the test subjects in a warehouse-like room and say the words for one particular character. At first, he obeys orders, but eventually goes a bit insane (because he doesn't want to be controlled) and attacks the other test subjects (to show that he can still think for himself, even when he's being controlled).

Thankyou so much, guys! Your advice was very helpful.