Martin McDonagh's play The Pillowman is filled with tragedy, and each of the main characters suffers from his own share of personal tragedy that affects his character and actions.
Katurian is a writer who was forced to listen to his parents torture his brother, Michal, night after night. Katurian's writing becomes dark and violent, and eventually, he smothers his parents with a pillow to stop their abuse of Michal. Michal, of course, is the one who had to endure those years of abuse. He is mentally disabled, and he ends up murdering two children, following his brother's stories. Katurian smothers Michal with a pillow so the latter doesn't have to endure the torture of being executed. Katurian himself is executed at the end of the play.
Ariel and Tupolski are the two officers who are trying to find out who murdered three children. Ariel has a tragedy of his own, for Katurian realizes that he was probably sexually abused by this own father. Tupolski is a coldhearted person with little emotion. He cares nothing about executing Katurian by shooting him in the head. He is completely detached, suggesting that he, too, has struggled with something in his past, something that has made him dissociate with people.
You will have to decide for yourself which character has faced the greatest tragedy. Just be sure to back up your answer with reasons drawn from the text. If you choose Michal, for instance, you might point to the years of abuse he endured. If you chose Ariel, you might talk about how his abuse has scarred him for life.