It's obvious enough: an extreme pro-life attitude of "killing microbes is a sin!" would involve considering Kirk's profession one of murdering.
(Which it is in objective fact anyway: no law allows for the killing of an enemy soldier on both sides of the exchange
- it's always murder from the POV of the victim. There's no law that says "Foreign citizens wearing uniform can freely kill our soldiers"; there's just the custom of neither side prosecuting the other's murderers after the war is concluded, because that wouldn't be practical. At most, key figures such as high-ranking officers may get prosecuted for conducting the murder campaign.)