Orcs (and goblins along with them) often get thrown to the wayside in games. They are beings that are safe to kill. An ontological evil that is too simple for its own good. Like culling a wild beast.

However, the most prominent explanation comes from "The Silmarillion," which states that the Elves believe the Orcs are corrupted Elves (an account that helps explain Adar's origin in the show .) ...