The Martyr
Very simply, from what is described, the Sin-Eater is a position that provides ample excuse to sacrifice the undesirables of any given village. Those who simply humans deem are unworthy of God in some way, and so very often do pay for it with their lives since one) they're eating off corpses, two) if they have that person's sins than are they now marked as sinful and deemed "acceptable" to further shun and even attack? People can be-- savage and stupid in so few words. And three) they aren't even paid well for the work, at best get a meal infested with maggots and flies for their troubles until they're back to starving by breakfast time and no one to give them the time of day.
The Sin-Eater supposedly such an important task, is not left to the "worthy," not those with souls deemed saintly or innocent. I doubt they think children should be spared for their imbibed purity as God's favorites, God's most precious creations and angels among humans. I doubt such thoughts of who may die or be ill crosses their minds in order to spare those people the strain. Rather, who "should," be ill or dead.
Much more likely is that the Sin Eater is thought of in the ways of virgins sacrificed to mountain gods in Edo Japan, perhaps beautiful but more likely little girls deemed unsuitable for marriage among the boys and demonized by the adults and only family to defend her if she's lucky. Or the unlucky child in 'The Ones Who Walk Away from Omelas,' who were blind to a single child's constant misfortune since it forfeits their utopia, which is frankly a sickening principle. It should be that the collectives are safe, that a society is loyal and serves the many-- as many as it can-- and do good by its people.
In any fair world, in any one that supposedly reveres their God as much as they fear him the Sin-Eater should be exalted and do that duty willingly and for selfless purposes if they wish to preserve the meaning in the first place. Then otherwise what kind of fair is it if a person already sinful is tainted with the sin of greed and vanity? Or better yet, the entire ritual poisoned by prejudice, disdain, and apathy?