Not quite sure where to begin with this thread, since it's kind of all over the place. I don't think that DnD fantastic racism exists because of inherent racism, I think a better explanation is that DnD is very derivative of mythology, and mythology all over the world is full of "always or mostly evil" creatures. It has stuck around due to DnD being conservative with its lore and due to it being a very easy tool for creating plotlines for your saturday night beer n chips game. Of course, I am not saying that this justifies DnD's current treatment of it.
Also @veti's posts, you're assuming a consequential form of morality which is pretty contradictory to DnD's, which seems to be more deontological in approach.
Pretty sure the word you are thinking of is actually "rational." Sentient refers to feeling, not reason. For example, animals are sentient but they do not live their life via intellect, or at least no where near the way we do. You also seem to view rationality as being black and white, when really it's shades of gray. Just because you allow an emotional belief to have an effect on your thinking does not necessarily mean you are entirely irrational, and would not justify you having your life controlled by a warden.If we cannot expunge this instinct, then we humans are not sentient, having failed the fundamental requirement of sentience, the ability to overrule our instincts with our intellects. WE would then be the monsters, acting on blind instinct to harm and destroy, and in need of the appropriate wild-life management from truly sentient wardens.