My son-in-law has spent 17 years in the service, and been out of country for 8 of those. He loves the Lord and he is born again, he can’t seem to accept the fact that God has forgiven him for the “killings” he did or the fact that he was in charge of other men and couldn’t bring them home safe. I know that the blood of Jesus is bigger than any sin, how do I get him to believe it?
From my understanding of Scripture it doesn’t sound like your son-in-law has a particular sin issue. The Bible contains several instances where the Lord commanded His people to kill their enemies, and He could not order them to sin. Also, most people believe the commandment against killing pertains to murder, not warfare. In addition, Romans 13:1-2 says we are required to submit to the governing authorities, which would include a soldier’s superior officers.
Even so, God knew all the sins of your son-in-law’s life long before he was born and when Jesus went to the cross he died for all of them (Colossians 2:13-14). As soon as your son-in-law was born again he was forgiven for every one of them, even those still in the future. What that means is that God has already forgiven him, but he has not forgiven himself.
From what little you’ve told me it sounds like he might be suffering from “survivor’s guilt”. It may take some time with a professional counselor for him to get over this. Hopefully you can find one with experience in this area who is also a Christian.