jack, for his sins, was sent to hell. he argued that he only lived that life to survive but God wasnt hearing it. he pleaded and pleaded and eventually God's child (unnamed but he does exist canonically) suggested Jack could be used to hunt other sinners down. Jack agreed to this under one condition: he'd never have to go after his family members or descendants. they agreed, and that's how he become the skeleton he is today. (he was already called Rattlejack before he died)