Never seek revenge of any kind. Seek peace and strive for constant self-improvement. Wasting your time trying to get back at someone who hurt you will only further prolong your healing and compromises your self-respect
This proverb is a reference to Claudius killing his brother. The poison (the axe) was only the cause of old Hamlet's (the tree) death. He still remembered that his brother killed him and he came back as a ghost to tell his son to avenge him.