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.
Fortunately for us, you can't kill someone with a CPU cycle. You need to use a servo for that. Which, while quick, are far from instant, and will always be limited by basic physics like thermal management.
Urgh, yes, that's so annoying. Skynet drones won't pepper the ground behind the hero with bullets while flying low and slow, they'll land every shot on target in milliseconds, before any human even sensed there was a threat. And they won't have a standards-compliant USB port to take over control either.
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.
Machines don't currently, in any form, do their own maintenance. A killer robot army would utterly dominate the battlefield until it broke down and ran out of consumables. It would then completely stop.