I think that one of Harry's biggest mistakes in the whole scenario was not bargaining harder with Voldemort before they entered the corridor. It was clear that he had some leverage over Voldemort there, Voldemort needed him for some unknown reason. He should have been less fearful for his own life (since Voldemort apparently needs him), and tried to barter for limits upon Voldemort's future reign of terror should he succeed. For example, if he agreed to help, Voldemort would need to promise in parseltongue not to Kill, Torture, or Imperius (or have minions do it for him), more than X people per year. (Limiting to X instead of 0 might be enough to get Voldemort to go along with the plan, and Harry might be able to morally justify his assistance given that he would be saving hundreds of students lives that Voldemort was holding hostage, so the net result might be less lives lost).

He could have sold this to Voldemort as the only way that his moral compass would allow him to assist him: by making the scenario into a trolley problem, where Harry helping Voldemort was the option that cost less lives.