Hmmm.. maybe the power of a death sacrifice is proportional to the ability of a person to produce blood? That way a 70 cancer patient is just a less useful sacrifice.

And/or maybe the magic from multiple sources can't be added, so one pint from a hundred people is sometimes no replacement for all of one person's blood.

The criminals
So there's a couple of ways the law could "work" to be regularly sentencing a lot of people to death. They all basically come down to not having a good legal system as good legal systems typically results in crimes being discouraged.

The law could be vague (undue vexation), confusing (gnostic turpitude), or beyond one's control (vagrancy).

Another random idea
A person can ask the priesthood for a new identity, this will typically be a person in debt or a criminal or something.

The priests do some divination ritual thing, possibly leading to you being sacrificed.

If you live, you take a new name and are given a parcel of land. Your new identity is morally and legally free of any consequences of the old life.

When people talk about a person sacrificing themselves, they don't distinguish between the outcomes.