The power of a crossbow to cause mass casualties is pretty low. Even conventional small arms have limited potential when used as single weapons, and there is always a human deciding to pull the trigger, who might disobey illegal orders to kill the innocent.

When implementing an fully automated AI powered kill chain, there is noone feeling responsible for the actions of these machinations, because the emotional distance is too large. AI systems do not have the concept of an atrocity if built for this purpose. And the killing only stops when someone disables it - but i'd say that if someone was unscrupulous enough to have started something like this, he will keep it going just to ensure he won't be punished for it.

I'd say the actual potential to cause harm to innocents of such a system is much larger than the threat of nuclear war, because the latter can only be started by an actual madman who would have in most cases to fight his own staff who actually wants to live. The former i can see being implemented by run of the mill politicians and generals, or even some cartels.