On Arravis' original statement:
For the FR flavor Monk's of Illmater need to remember one of the classical lines out of the FRCS "Turn the other cheek". If the monk has the ability to do good in the immediate moment than that is the greater good in-her-case. She should help ease the suffering of others around her in whatever form her own skills match the situation. Part of being a monk, and I like the approach you mentioned (movie wise), is being acutely aware of your own limits and of the environment that you are a part of in space/time.
"To be suffering with another is to be the other; losing oneself, and the distinction that any suffering is not our own, is our truth." - Tao de Illmater
Illmater is not about needless suffering though, so your idea for the 10 days is a very honorable way of dealing with the monk's own inner demons. Walking the path of perfection through recognition of suffering is noble and difficult; Illmater does not expect his followers to be perfect in their pursuit of the greater good. Bringing those who caused the evil act (yes mutilating others falls in that category of evil) both serves the monk's thirst for revenge and serves the good. It discourages some criminals who see the miscreants brought to justice, and promotes local jobs.
I like the Inigo Montoya idea. Perhaps another one: the "evil-doers" are important officials in the area who get away with occasional acts of violence because they keep the city from falling apart (i.e. Lawful Evil power center). The monk keeps her peace because for her to act would tear the area apart, but during her 10 days hires the players to find out information which will bring the officals to justice for their crimes...
On the ethics, morales, and perception of good, greater good, and evil:
"No man walks with his feet 3 inches off the ground." - trans: those who are perfect leave this mortal realm we speak of and our discussions of good/evil are nothing compared to the reality they experience. We, as mortals, wish generally to do what is 'good' in whatever format we see it in. We cannot, and do not, see all possibilities and all probabilities of all of our actions all of the time. So we resign ourselves, through experience and maturity, to doing what we believe is good when we can.
There is no universal "greater good/evil"; the universe does not keep ticker marks of one good outweighing another. Terms such as these apply only on a biological/sociological level. In which case, saving the water to save the greatest number of lives is best, unless saving the life of the person trapped in the car will save more lives later (because they are a doctor, fireman, messiah of some sort, etc.). But there again one runs into the problem of perception and doing "the best one can".