FACT: John Marston bros it up with the devil, who clearly has the power to decide people's fate.
FACT: Bill is a lone-wolf badass that has lost his partner and has trouble making friends
FACT: John is a lone-wolf badass that lost his partners and has trouble making friends
FACT: John Marston's face:

FACT: Bill's face:

Bill is clearly the reincarnated John Marston, raised by the devil to fight the zombie infection (which he did in his previous life as a cowboy bounty hunter).
OTHER FACT: Joel looks and acts like John Marston too, meaning he must be John's great-great-great-great-great-great grandson. John was reincarnated as Bill to help his great-[...]-grandson kick zombie ass like he did himself.