I understand that the internet was invented/evolved as a robust distributed system that allowed communication specifically even when subject to attack or damage. Having a 'kill switch' is completely against the core purpose. as mentioned earlier, you just introduce an attack vector that was engineered not to exist. just get enought leverage against the killswitch operator and you can cause major damage.
I speculate that this is a major reason behind tiered internet. the kill switch can shutdown only the 'low tier' users of the internet, (punters, small businesses, the small fish, whatever). But, the top tier (military, corporation, gov, banking,power generation, big fish, ) can keep running unaltered (maybe even faster, and with more hardening). In this situation, the killswitch looks a lot more useful, or at least useable.