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?
Certain countries treat crimes of passion quite leniently. Whilst some laws have been repealed that would get you off the hook completely, you are still likely to do far less time. There is a difference between these kinds of murders and ones that are premeditated.
I just don't think you can look at it in a black and white way.