If the entire world was to be killed which champ would do it the fastest

resets for days

Karthus is the only one with a global ability that can damage everyone at once. Cho'gath would have to go around eating people one by one. Kindred's kit would actually be counter productive because her ult would be saving people. Killing people by other means would require a lot of time for her, not efficient. Viktor's ultimate could eventually kill the entire world, but there would be a long period of people dodging his cloud. Azir would just be able to push people around until they eventually died, but he'd only really be able to get 50 or so people max at once - and that is in optimal surroundings like a packed subway. Vel Koz could maybe do some damage if he could get his ultimate to shoot for longer and further.

But currently, Karthus would be the absolute best at ending the entire world since he could just keep ulting and eventually kill everyone.

summon Black hole and laugh maniacally. http://img.timeinc.net/time/daily/2011/1107/360_water_vapor_0725.jpg

All those resets

Don't you mean

What about with

goes completely mad and create a giant bomb that destroy the entire world with the potential to create a black hole and destroy the whole galaxy