"Only Death Can Pay For Life" is a notion that's been used several times.
Mirri Maz Duur says it after she
Saves Drogo, explaining that her unborn child died to bring Drogo to life. Make sense, death (Rhaego) paid for life (Drogo).
Arya saves him and two other prisoners. His reasoning being that she "took" three deaths from the Red God, and so she owes those three deaths to him.
Once again this makes sense, three deaths pay for three people who continue to live.
Arya kills Meryn Trant in the brothel. The faceless men know that she's killed someone and that it wasn't part of a contract. One of the faceless men then say that "Only Death Can Pay For Life", and drinks a poison that kills him.
This makes no sense to me, in every other context someone died so someone else lived, however in this last example two people died and not one of them was left to live.
Where is the life that
Meryn Trant's and the Faceless Man's
deaths paid for, that would legitimize the use of the quote in that context?