It's not so much software that's the issue, it's people taking existing tech and weaponizing it.
Software has been in weapon systems since the 1970's from guidance to target acquisition. It's already there, and has been for a while. Machine learning is just another iteration on what's already out there.
The world knows how to kill en masse for a long time now with NBC (Nuclear/Biological/Chemical) weapons -- and software wasn't a part of Fat Man or Little Boy.
But consider the challenges the US had in the middle east for the past two decades.
* Consumer drones dropping hand grenades.
* IED's using manual triggers.
* Trucks loaded with explosives blitzing checkpoints.
* Cars with explosives driving up next to convoys.
* Land mines.
You don't really need custom software, just take existing tech and get creative with it.