Bitter, can be talked about in the sense of taste or feeling. In this article, I choose to talk about it in the sense of human feeling.
At some point in everyone's life, a person must have been bitter with someone, himself, environment, or life as a whole. Life must have been unfair to someone that causes him/her to be bitter with it, sometimes the bitterness is with a Person, some other time, a person is angry with himself, probably because of his wrong choices in the past. Some are even bitter with the environment they are born into (country). At some point, persons are bitter with the supernatural being (God). I think, I won't be wrong if I say, everyone has been bitter at some point in their lives.
There is no quantity or quality of wrong that will make someone bitter, it could be something trivial like missing someone phone call, and he get bitter for it. People could get bitter by reading meaning to a simple statement that sound harmless on the face of it. The fact here is that, no one can guess what will make someone bitter.
Bitterness can get to the extreme when the person begin to think evil as a plot to revenge the unfair treatment that has been melted on him/her that causes the bitterness. Some even get physical(fight) when they are bitter. Some even threaten to stop worshipping a supernatural being (gods) when they are bitter, or feel it has been unfair to them. Some migrate out of a country when they are bitter with the government.
To be bitter at some point is necessary, because it causes one to ask questions, challenge the powers doing wrong, or challenge one self to make better choices or undertake a life changing venture.
I think bitterness can be better handle by the lapse of time. Time heals all injuries they say. I will suggest that, when one is angry or bitter with someone, himself or God, he should not take action immediately, because most decision made in anger, are not rational. People have committed suicide, murder or blaspheme God when they act in the heat of bitterness. But with the lapse of time, the anger and bitterness would subside. By then the person will be rational to ask and take better decisions that will further bring development to one's life.