Another way to look at this (link me if someone's said this already) is as repeated matches of the Prisoner's Dilemma. The winning strategy tends to be "Start nice then match what the other guy does." So if Harry's considering this Hermione/Draco thing as the beginning of Harry vs. Voldemort (prophecy baby wouldn't count to him), no one died, it's fairly obvious that no one was meant to die (well, after ten years Hermione might have been dead, but that wasn't a given, Bellatrix and others from the last war have survived a decade), so Harry can hang on to his "no killing yet" stance for now.