If most G20 nations ban bitcoins, would that effectively kill bitcoin?
My thinking is that (i) G20 nations are democracies, and (ii) Bitcoin is a useful service. If it's useful to people, and people want it, they'll vote for it and it won't get banned. 

If your country bans something useful to the masses, it's not a democracy. It will be banned only because it lost its utility, and if that happens, there will probably be something better replacing it.