The Minchas Chinuch has what sounds to me like a paradoxical chiddush with respect to the mitzvah of saving a nirdaf from his attacker (#600). The halacha is ain holchin b’pikuach nefesh achar harov. For example, even if the majority of a city is not-Jewish (and m’dina d’gemara one cannot be machelel Shabbos for a goy), if there is any doubt as to identity of someone who needs help, one should be mechalel Shabbos on their behalf and not assume based on rov that they are a goy. Similarly, the mitzvah of saving the nirdaf only applies if he is Jewish, but, writes the Minchas Chinuch, if there is any doubt, even if the majority of the city is not-Jewish, ain holchin b’pikuach nefesh achar harov and one can do whatever is necessary to stop the rodef, even killing him.
Why does this make sense? True, in a case of safeik, even if there is a rov, we act to preserve life, but in this case we are sparing the life of the nirdaf at the potential cost of killing his attacker, the rodef. Why would we not invoke the safeik/rov and argue to spare the life of the rodef on that basis? And even if it is inevitable that one of the two will die, shev v’al ta’aseh adif, what gives the onlooker the right to interfere in a situation of safeik in a way that will cost a life?