The old testament and God commanding the Israelites to attack the Canaanites has to do with Genisis chapter 3 and the sons of God narrative. God is trying to clean up what is left of that "giant" blood line that was wrongly created. Goliath was a symbolic ending to the Nephilim tribes.

This is just my opinion but Romans 12:19 also has something to with Heavenly judgement. If you're the hands and feet of God confronting evil, don't try to actively be God by sending that person to eternal separation from God. That judgement belongs to the LORD.

Otherwise wouldn't a soldier have to give up heaven to be a soldier?