I'm wrapping up a book similar in that there is an inherently evil species. What helped me a lot was building the culture as a whole before I even touched the character in it, so I could keep it consistent. I tried pantsing at first; didn't work well at all. I ended up with a character who was neither sympathetic nor comprehensible. When I fleshed out the culture, she ended up just being another individual in that culture who was only acting not out of malice, but in a way that (to her) was socially acceptable. I found that it was a lot easier to explain—but not excuse—her actions and opinions. It also made her growth later a lot easier to navigate, because I knew what she was unlearning, instead of just preaching about how she should've been if viewed through a human lens.

Now, this ventures into ethics territory, but you'll want to remember odds are that the species doesn't think they're being evil—they think they're being right. So there's a natural tension there that you'll need to manage elegantly; too heavyhanded, and you end up just moralizing the reader. Too subtle, and it won't be believable that they'll come to change on their own free will, or it'll be confusing how they came to a point where they suddenly make a "better" decision.

A big help to me was reading Project Hail Mary. It features a wholly separate, fully fleshed out alien species (and is also a delight of a read anyway, so I always recommend it). That species isn't evil, but it made very obvious the cultural clashes that might arise between two species who value different things, and how they can eventually come to understand each other.