According to who's law? When countries are at war, who gets to decide the difference between killing and murder? And if it's the winning country, is it a Godly idea that might makes right? That's why these things are left up to the conscience of the believer.
I personally don't believe Christians should be involved in a country's military, because I believe most of the killings in unjustified wars (and pretty much all wars in the last 50 years were unjustified) rank as murder. However, I also know many Christian military men who I respect as God-fearing. And where would we be with the world's current pedophilia and child trafficking pandemic without the military, irrespective of the past 50 years of wars? Likewise with taxation being theft.
As such, even things some might see as murder and stealing are to left to the conscience of the believer. While I might say murder because the war is unjustified, you might say killing because the other country's military wasn't just going to step back and let the country get pillaged. Ultimately, we are each accountable to God for our actions, but we're certainly not under the Old Testament.