It is highly unlikely that 750V can deliver instant death. I guess
you'd need at least 50kV for that under normal conditions.

There are, however, people who have survived much higher voltages, but
many have taken lasting damage in the process.

There are also people who have been killed by 50V.

It all depends on the duration of your exposure, the impedance of the
return path and the part(s) of your body the current flows through.

5 milliamps is sufficient to permanently stop your heartbeat. So you
could theoretically kill somebody with a watch battery. You'd have to
dig the electrodes into the heart tissue first though, so that sort of
thing is unlikely to happen by accident.

On the other hand, there are people whose hands or legs have been
roast to cinders, but who survived because the current didn't get
anywhere near any vital organs.

The safest advice is, never mess with electricity.