[@Alfhedil] Well, lacking any clue of what anyone else is thinking of, and I'm both flexible and coming up with this off the cuff...
I'm of two minds at the moment, as I usually am when given a setup like this. Depending on what other people are throwing up, first thought is to play up as someone on the edge of heroism and villainy, leaning into villainy due to some fatal flaw in their powerset that leads them to the conclusion that they [i]can't[/i] really do good with it. So likely some powerset that is archetypically villainous: poison, darkness, or perhaps something linked closely to emotions and give them just the wrong kind of temperament to use it nicely and not enough self-awareness to notice that it's emotionally connected.
Having written that now, yeah if that's the broad angle I went with I would go with linking it into their emotions and have it seem to act on its [i]own[/i] from the perspective of the character despite the fact that it's just them lashing out emotionally. A danger to themselves and those around them just because they have no real control of themselves.

The alternate is to basically go the exact opposite in terms of personality. My brain isn't feeling the middle ground right now, so while yeah sure the middle ground exists, going for pure, unadulterated idealism. A hero because doing good is good. It's the right thing to do. Don't know about powers yet on this end. Details tend to just flow in on me when they feel like it, see above for an example.