His being forgiven by a Catholic priest did not excuse him from being hung.
The priest forgave Hoss for the eternal punishment that he would otherwise have endured. In other words, Hoss was hung, but because he repented and received absolution and viaticum and accepted the temporal punishment for his crime he did not go to hell.
Faced with his horrible crimes one might still object and say, “But you still saved him from hell. He should have gone to hell!”
One can certainly agree that Hoss should have endured even more punishment than simply losing his own life, but this is where Catholic doctrine is not only merciful but just. It is just because we believe in purgatory.
Purgatory is the place of purification, and we are taught that the fires of purgatory are painful. Furthermore, if the soul, who is snatched from the fires of hell is a terrible sinner like Hoss, then the fires of purification in purgatory are intense and their time there is long and hard. They will make it to heaven one day, but it will not be an easy or a quick journey. It will be very long, very hard and very torturous, and it will be just.
Dante’s image of purgatory is that the punishment fits the crime. Perhaps for many many ages Hoss will experience the loss of loved ones, the hunger of starvation, the torture of beatings, the loneliness of the punishment cell, the deprivation of losing all things, the terror of the gas chambers and the heat of the ovens.
The final thought is that if he was truly repentant, then he would endure these pains of purgatory with forbearance and faith–knowing that he is getting what he deserves, but also knowing that he will be saved at the end.