I'd recommend extreme caution using deadly force in defense of a third party in a situation where you don't know the "agressor" or the "victim." You may not know all the facts. For example, how do you know that the "agressor" isn't a plainclothes or off-duty LEO? Or how do you know that the "victim" didn't instigate the incident?
Of course, there may be situations where it's clear that the victim is in need of aid (e.g., a man mercilessly attacking a woman) and use of deadly force may be justified, but extreme caution and circumspection is needed.