The first rule of Christianity is, “I am the Lord thy God. Thou shalt have no other gods before me.”

There is no probation against killing in general in Christianity, only murder, which makes an appearance as the 5th commandment. Murder being unlawful and/or unjustified killing, so clearly wartime killing is a-okay in Christianity.

Now, you could argue that “love thy neighbor as thyself” might implicitly forbid killing, but if you start looking to Christianity for morality or even simple consistency you’re going to have a bad time.