In trying (but failing) to kill Harry Potter, Lord Voldemort left a part of himself in Potter. From then on, they share part of their karma. Hatred breeds hatred. To free himself, Potter had to destroy the hatred within himself.
In J K Rowling's fiction "Harry Potter" series, Lord Voldemort attempted to kill baby Harry Potter (Book 1). His spell backfired. As a result, Voldemort left a part of himself in Potter. Neither Voldemort nor Potter realized that Potter became one of Voldemort's hocruxes. That is why Potter shared part of Voldemort's experience. He also shared Voldemort's hatred of each other, to the extent that "neither can live while the other survives", as part of the prophecy. In the end, Potter had to let that part of him die before he could free himself from Voldemort (Book 7).
After the Voldemort part of Potter died, Potter no longer attempted to kill Voldemort. When confronted by Voldemort, he cast Expelliarmus, the disarming spell, when Voldemort used Avada Kedavra, the killing curse. Voldemort killed himself as his spell backfired.
When someone tries to hurt you, he leaves part of him on you. Very often you share the hatred that he left on you. Sometimes you want to hurt him in retaliation. Until you clean up that part of you from your mind, you'll always suffer. In other words, you'll have to forgive in order to release you from the suffering that he's caused.
Our acts all affect each other. Every action leaves karma in this world. Unfortunately, many of us can be troubled by criticism. Most of us will be troubled by physical assaults. Sometimes even the calmest or most intelligent people can be troubled.
One final observation: backfiring happened in many places in the book. If and when you try to hurt others, no matter how righteous you think you are (as Ron Weasley did in Book 2), you often end up hurting yourself most.
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