Bitterness is a root, which means it works underground and shows itself only when it has already spread. Resentment starts as a private grievance a man nurses in the dark. Feed it, and it grows a system, a whole account of why he was wronged and owes the world nothing. Watch for it early, because a root pulled young comes out clean.
What makes it dangerous is that it never stays private. The embittered man poisons everyone downstream of him, his family, his friends, the people who did him no harm. Contempt is contagious. Tend the ground of your own heart with care, catch the resentment while it is small, and you spare not only yourself but everyone your life touches.