He appears to have been a gambling addict, going deeply into debt to a bookie in West Lake. He eventually emptied his and his wife's accounts and stopped paying his mortgage, allowing his house to go into foreclosure.
He later arranged to meet with his bookie's enforcer at a cabin in Chester's Mill, but had no money and attempted "to renegotiate." He was given one additional day to pay, but instead pulled a gun on the man; he was promptly disarmed and the two wound up in a fistfight, during the course of which Shumway was accidentally shot and killed by his own pistol. In truth, Peter deliberately let the enforcer kill him, effectively committing suicide by opponent.