Let's say it has to be the death of someone above the age of 30. They have to either volunteer or be a murderer. Since we're dealing with hypotheticals there is no way one could fake having a volunteer or murderer. Otherwise the ritual would not work.
The power received would be key knowledge that only the opposite side(s) have, given to those who perform the sacrifice.
There is no way of knowing what the information will be, only that it is very useful to them.
Would you be willing to use this weapon?
This is one of the main arguments against capital punishment, and I do agree with it, but it isn't the most important one to me.
I just think it is wrong to kill someone because they killed someone. There is also the context in which they did it. Even if we're saying that there is no doubt they killed someone, I think we would all have conditions under which we would sympathise with certain murders over other ones.