Go Postal - Go postal simply means to become angry, or become too much or to behave in a very aggressive way or very violent manner.
Let's talk about the samples.
1. When Ram failed and showed his results to his father, his father went postal.
2. I was not in a good position, because I had lost the contest, and I had gone postal.
3. He always teases me, I think I must give him an answer, going postal with him once at least.
4. My mother always goes postal when I do not study.
5. He is a very big brigand, and always goes postal on very small things.