That's convinced me: they should care, so there has to be a mechanical reason for them to do it. I've played in too many games where the 'heroes' stopped being heroic because, really, there was nothing to stop them and no incentive for them to be good. This is a game about being a hero, so it needs more than just player buy-in to keep that going: taking a life, or just standing by and doing nothing while someone dies, should always be a hard, desperate choice for a superhero.
I think rather than fiddle about with scales and measures, I'll create a fixed penalty for being responsible for a death, a real 'the world hates you now' deal, and let the 'Dark Avenger' have a move that makes them exempt from that penalty. There's already a model for it in the hack, a face death move which lets you decide whether you want to retire your character or keep playing them when they fill in their last harm segment. Maybe face judgement as the flip side, for when you do something truly anti-heroic?