Low: In canon, there was this whole 'genophage' thing he contributed to for the sake of avoiding galactic war/genocide. It was preeeetty bad. In-game, he hasn't done anything really bad yet, although he is going to be kicking himself for not shooting Boyd one day.

High: Sacrificing his life in the third game to undo said genophage and grant new hope to the species he doomed. In C&C... maybe treating an Import kid beaten by Vulcanus agents last year? Or defending the Clinic and continuing to treat patients during anti-Vulcanus riots in the City.