[Advantage/Disatvantage] Designing a gameplay mechanic

[Anaxim]

I was running a one shot with a watered down GURPS about a month ago, for which I created a cleric like character (TL3 campaign, high fantasy) with some twists on the original D&D concept.
Basically, the character does heal, but the powers that be make the healing art a lifestyle. A Healer (The actual title and name) starts, say, at 0 purity. Their efficiency is based on how pure they are, measured by healing and helping in contrast to harm and murder.
The way this works, is that certain things raise purity above the 0, causing the Healer to gain a slight benefit as well as be eligible to further advancement. The way they do that is when the Healer spends less on hitting, hurting, maiming and killing and more on healing and benefiting - the most powerful healers do nothing but healing: friend and foe.
Purity drops, and causes spells to lose levels and, eventually, Magery to turn off. Murder (All types of killing, self defense included) instantly turns Magery off until some manual healing and other beneficiary things are done. Anything less gradually weakens the spellcasting.
What I am unsure of here, is how to handle it. Vow isn't it, since the lifestyle is about principal and not "law", though some Vows can be derived personally. It's no form of Duty, since this is about choices. I can't think of other options that don't fit but are in the book.
I would like some input and assistance in crafting this gameplay mechanic.

Feel free to ask questions about anything unclear.

[Refplace]

I doubt I would build it as a disadvantage but more likely limitation on the power.

[Dalzig]

You could use the Clerical magic option on B242 and have the Purity replace the "does my god grant this spell?" part. If you have a low purity, your Power Investiture/Magery has a decreased effectiveness. If you have a high purity, you gain extra levels of PI/Magery.

[Anaraxes]

Pact limitation.

Depending on your power builds, Talent bonuses or penalties.

[Anaxim]

Pact would work, but only on the "do not murder" clause. Beyond that, it's too extreme since there isn't any gradual effect.

Additionally, this is a form of Magery - call it Life Force if you must - and nothing to do with actual religious connection.

Skimming through relevant seeming parts of Thaumatology yielded nothing.

[Anaraxes]

You could have different pacts for every level of Magery / Power Investiture (if that's the way the level of power is indicated). "Do not kill" applies to them all. "Do no harm" just might apply to the top level, knocking you down from Magery 3 to Magery 2.

[Anaxim]

I think the book-keeping on that might be a bit much, especially given that Pact considers itself as directly causing ineffectiveness. The precise percentage that is taken off per Pact is different between turn off and decrease.

[Not another shrubbery]

Originally Posted by Anaxim

Pact would work, but only on the "do not murder" clause. Beyond that, it's too extreme since there isn't any gradual effect.

Pact works fine for gradual loss of abilities also. See Powers p23 (the Required Behavior and Power Modifier section).

[Anaxim]

The gradual defined there isn't quite the gradual I meant.
Call my meaning... tiered.

Murder - complete loss.
Profound injury - level loss.
Plain injury - skill levels loss.
harmful intent - next spell level loss.

Sort of flip it for good deeds. The afterglow for good deeds stretches upward to a day unless maintained. Bad deeds balance out the good and can only be undone by doing good. This is a flexible and more immediate mechanic. It is supposed to only be effective temporarily and turn the cookie cutter cleric "Hit the enemy and heal the friend" into a complex lifestyle not quite fit for adventure. (Adventuring Healers often take breaks to improve upon their abilities.)

EDIT: Mitigation of problem attempt: Which is more adverse - the penalties or the bonuses?

[Kuroshima]

Pact is the way to go. I would go with a 5-10 point Code of Honor. Not following causes loss of "purity" and being very strict can cause an increase of "purity" that gives minor benefits (For example, each point of negative purity gives a penalty, while every 5 points of positive purity give a bonus, up to +4 at purity 20. Each day, positive purity decreases by the bonus it gives, but negative purity remains with you until you ritually cleanse yourself/do good deeds). The character only actually takes the disadvantage if it's appropriate for him (i.e. if he cares about the CoH. If it's just a required behavior that he can discard at will, at the loss of some metaphysical power, the he still has the pact, but he gets no points for the disadvantage)