The Monster With A Missing Heart

There once a boy who was a nice, friendly and innocent kid who has a great family and nice life. He thought he will have a great life but one day some random people kills his parents and kidnapped, they then force him to be the greatest emotionless hitman by doing unspeakable training and breaking his mind. As years goes by when he is a adult, he became a kill machine without a heart and gets paid to kill people who has higher power or they don't like them just like a assassin. Would he spend rest of his life as a killing machine or maybe goes back who he was before maybe someone will help him?