Bitterness is a prison-house. When a person finds themselves becoming bitter, they go through three stages. They speak sarcastic words. Then they think evil thoughts. Finally, they act on those thoughts and commit evil deeds.
Lets watch Joseph's brothers who were so filled with bitterness toward Joseph that they not only hurt Joseph, but their father as well. Because bitterness cares little for the feelings of others.
Let's not let this happen to us.