If you are going to believe the garden of Eden story, the very first thing God teaches adam and eve after getting kicked out is warfare.
God shows them spinning swords...
adam.
"holy shit! eve look at those spinny things bet they could do some damage"
eve
"Wow they spin really fast, but adam you can never get them"
adam
"yeah i know.....but it gives me some great ideas"
God showed man how to kill!