It would probably be easier to simply stomp on the victim at the base of the skull without even bending over. Breaking the neck with your hands is, almost always, a non-start. It might be possible to achieve the same result with a well placed hand strike, but that wouldn’t be nearly as brutal.
The other, arguably, more horrific option would be to run a screwdriver, icepick, or other similar tool through the base of the skull into the brainstem once they’re in that position. It’s not exactly the same as breaking their neck, but it’s certainly a brutal way to go.
In general, I’d recommend you check out our The Only Unfair Fight tag for more relevant ideas and concepts.