I wonder whether Orcs are capable of 1) introspection & self-evaluation, or 2) conceptualizing "evil" or "wrong." I could be wrong (but not evil), but I think they're not operating intellectually on that level. I believe they're rooted at a "pain/pleasure" level, and not intended to be truly "sentient." That's a little more subtle than JRRT's symbology permits (IMO).