He wants to revenge to those who got his dream job instead of him. He wanted a job in FBI many years ago, where your father is working. He didn't got accepted there, so he got mad. Really mad. He felt urge to kill every person who got the job instead of him. Including your father. Now he wants to kill your whole family. He got a job as a body guard. It's perfect for him, so he can get closer to your family and especially your father. But you are his first target.