Sometimes I can call other humans evil when they do something I find particularly abhorrent and malicious, but that's subjective isn't it?
Yeah that's a common idea. It can seem like ethics are an expression of preference or opinion, like someone could say... "eating that slice of chocolate cake was the most evil thing possible" but shouldn't that comment still track onto reality? We don't need to process the statement very much to know that's not the most evil thing possible but pretty clearly far from it.
Or... take something that doesn't even involve a moral agent: "Did you see how that rock fell? Pure evil". Someone saying that would just be incoherent, it makes no sense at all, and so it's not defensible. Even when someone says "Oh that's just my opinion actually"-- it's the wrong kind of expression because it pretends of preference when the thing being talked about is a description of something not entirely subjective.
The reason it doesn't look like it can be subjective is there's also an objective description of what is happening morally. That word has a relationship to concepts like wellbeing of conscious things, extreme suffering, etc-- and not to rocks randomly falling with no one around to suffer from them in any meaningful way. That tells us we're talking about facts.
Someone could say, "Chocolate cake tastes good", and to someone else, it could be that chocolate cake tastes awful. That sounds similar to a moral statement: "It's great to torture someone". To someone else, it could be that the idea of torturing a person is horrifying. Both do express values, preferences, subjects, so you're right that it looks subjective. But in the cake situation, there's no fact of the matter about what desserts taste good and which don't in any big picture objective way. But with moral statements, since ethics has an actual meaning behind it in a way that "taste" doesn't, there is a fact of the matter even in the presence of subjects expressing their personal values
I think the important thing to add here is evil can't be blamed for being evil because it didn't just decide to make itself(imagine the worst person alive right now, how unlucky would it be to be them? There's a reason no one would trade shoes with them unless they were suffering from some similar confusion), it was that way or became that way not in any free (blameworthy) way