How to make a good yandere character?

I'm writing a story romance isn't the main plot but so far as u was writing this character he's starting to seem like a yandere and I was wondering how to make him better.

Like my fl isn't a romantic person but the person who loves her was willing to help her destroy her country.

So far I believe I have like three yandere in the story. But romance isn't the main plot but I'd like to write them better without making them boring