Maybe this is how "being a member of a group which slowly shifts towards evil" feels from inside: Increasingly realizing the importance of loyalty, and that fairness is not as important as it seemed once.

So when you notice yourself thinking: "well, technically this is not completely fair, but our group is good and we do many good things, so in the long term I can do more good by sticking to my group than by needlessly opposing it on a minor issue", you have an evidence of your group becoming just a little bit more evil.

(To be precise, "a little bit more evil" can still be predominantly good, and can still be your best available choice. It's just good to notice this feeling, especially if it starts happening rather frequently.)