Your experience already tells you that a human being is different from an animal. More proof than your own experience is not necessary, you see it every day, don't you?

Check your real life experiences. If you personally know someone you classify as a "gray being", you have not yet managed to take a different view of that person. Because, for example, you are not curious enough on the part of this person whom you have not yet experienced or you have not managed that this person can surprise you. In order to really get to know someone, you could spend a few days alone with them, perhaps go on a hike, go camping or build something together etc. It should be possible to penetrate the other's shield in a good way and let him dare to show himself as he does when he trusts someone.

You can't ask anyone to prove that they're not gray. You can only control your own view and what it feels like to have an experience that is different than your expectation suggests.

If you haven chosen another way of life, give people the time to walk their paths in their own pace. Ten years back it was different for you and it'll be different as well in ten years. Wanting to be flawless often creates more suffering than good. Then you put yourself on a strong chain and tend to be unforgivable to yourself.

Why anyway concentrate on which you find wrong? Why not concentrate yourself on things where you can feel you have a good impact and do what you can do?