Some weapon systems arm themselves according to certain conditions, altitude, time from launch, etc. I think one could argue that some missile systems are already robots that "pull the trigger".
Best decision: don't build it, if it's already built don't launch it, destroy it as soon as possible.
Short range devices sure, but they still require a human to point and launch. There's not a margin of error there that will divert a missile from an armored vehicle to an elementary school.