John Kramer was a normal man who was married to Jill Tuck. She ran a health/addiction clinic. While she was pregnant with Gideon (little Jigsaw) One day while closing her shop an addict named Cecil tricked her into letting him in to get more drugs. When he tried to leave he slammed the door into Jill's stomach killing Gideon.
Soon John was diagnosed with a melignant tumor (cancer) of the frontal lobe of his brain. Knowing the exact time of his death changed John. He drove over a cliff trying to kill himself but somehow survived. He there vowed to test the human will of survival because, "Only when faced with death can you really tell if a person has what it takes to survive." After his transformation he soon adopted his "son's" toy puppet BILLY and his tricycle from when he was a kid into the horror symbol that we know today.