What value of voltage can Kill?

A man standing up to his chest in water was once killed by a voltage of only 50 volts (he was a working in a shipyard). This is exceptional. In general, the voltage would have to be above about 100 volts to kill. For low voltages like 100 - 300 volts, death is usually caused by the electricity shocking your heart into a state where it vibrates instead of pumping (fibrillation). It can't easily be started again without special equipment. For higher voltages, the heart usually stops completely but can be started again with thumps on the chest. Im this case, the usual cause of death is internal burns.

A shock across your heart is more likely to kill. The electrity travels from whatever you touch that is 'live', through you to electrical earth. If you are not earthed there is not a problem.

If you touch something live with one hand and the other hand is earthed, the shock goes across your heart - bad. If you are earthed through your feet the shock goes through your legs and is less dangerous (not exaclty good, but less bad). This is why electrical engineers like to keep one hand in a pocket when working :-)

Strictly speaking VOLTAGE of ANY value will NOT kill.
It is possible for a human body to be connected to a 1 million volt generator without any harm whatsoever.
Using a suitable voltmeter it would be possible to measure this on the persons skin.
Of course this is relative to something that is not at 1 million volts - usually ground or earth.
Providing there is NO current flow, or a very very small amount, then it is safe.
This is achieved by being insulated from anything that would allow a current to flow - usually rubber boots being the minimum protection.
If this were not the case then shuffling across a nylon carpet and discharging the static electricity created into a grounded metal object would kill you.