The crux of the problem comes down to portraying a "race" of creatures or entities, if presented as sapient and self-aware, as inherently "evil". Zombies are built to prey on the living and aren't (in most cases) thinking entities. Tyranids in Warhammer have no good or evil intent at all, and most have no individual sentient minds. They exist to perform a function that happens to be destructive to everyone else.
When it comes to stuff like orcs, it doesn't really require a lot of work or explanation at all to quickly draw the difference between a malignant culture, and individuals. Warhammer orks are maybe the thorniest to deal with, because are imagined as creatures who are intelligent, but need to fight to live. Therefore, they have created a culture which strongly advocates the only thing orks can do is find people to fight. An interesting quality there, is Warhammer tends to not portray orks as being "evil". They're happy and excited about fighting, and aren't trying to murder people because they hate anyone. But you can see how the whole idea might be seen as iffy.
There generally isn't a problem when you're writing about a specific evil being or singular entity. Explanation isn't even necessary if, as some have posted above, the evil is in service of mystery and horror. Individual things can, for whatever reason you imagine, become "evil". The problem is usually when you invoke the concept that races can be intrinsically evil.
Some fantasy traditionally gets around this by having races being literally possessed by an "evil force" at the cosmic level. Then they are essentially demons in the classic mythological sense. This can work, if demonic comparisons are put to the forefront. D&D has a large number of intelligent monsters which are part of a species, but are also supernatural beings and by nature malevolent. Beholders are a well-known example.