I was a police officer for nine years and I come from a military family. There is a great difference between murder and taking a life in self-defense or in the defense of others.

King David was a man after God's own heart according to the scriptures, but he was among the mightiest of warriors. This is how I had to reconcile it in my heart and I know this is how many christian soldiers do as well, especially when so many would not have chosen to be put in situations they find themselves in.

I hope this helps.

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Romans 8:6 - The mind of sinful man is death, but the mind controlled by the Spirit is life and peace