In speaking of soldiers (police nowadays) He says "They don't carry the sword (gun) for nothing." He set them up to keep peace in the land, and to hold people accountable. It is logical to assume they had training with these swords. Martial arts is another form of protection, and just as God authorised swords to the trained, anyone entrusted with such things must use them in a godly manner. David killed his tens of thousands in war, but did he kill civilians? No.
You need to decipher what is right. If a man slaps you- that is a spiritual battle. If a gunman enters a school- that is a physical battle. If a man tries to kidnap your child, and you can use martial arts to prevent that from happening, which is worse- for him to have bruises, or for your child to be raped, decapitated, and thrown in the river? Who here wouldn't call 911 in an emergency?
You need to disern between physical battles and spiritual battles. Jesus on the cross was a spiritual battle. David killing in war was a physical battle. Daniel in the lion's den was a spiritual battle. The israélites crossing the parted sea was a physical battle (you don't always have to be physical in a physical battle, you can duck out of the way).