The speed at which computers run is beyond human intuition and beyond the speed for humans to respond. A killer robot will kill everybody in range practically instantly. It would be nothing like what movies show.
The only real question is whether the machines think one bullet to the head is most efficient use of resources, one bullet per organ, or an equivalent mass of non-depleted (they don't care about radiation, the biosphere is not their problem) uranium to the target biomass.