u/Omyo-wa-mou-shinderu

For background, in my book, I’m trying to write a twist antagonist to the main character and the other side characters of my book. But at the same time, I want to subtly hint at them being an antagonist at the very start, but not too obvious for it to be flagged as foreshadowing by the reader, and only seen once they go back and re read…and my main idea for this is to write the side character in question’s reactions, and their tendencies in a way that would hint at them evolving into the antagonist I’m envisioning.

So, my question is, Is there any other great ways to hint at a character being an antagonist?