You have many options! What kind of functionality would you like your kill command to have?
Easy mode: 99999 damage thunderclap with targets set to "self, vulnerable, invulnerable"
Moderate mode: 0 damage thunderclap with a trigger on cast that uses the "Unit - Kill" or "Unit - Remove" command
Hard mode: Trigger that watches player chat for something like "-removeunits" and kills/removes all units currently selected by the player
WTF mode: Trigger that catches when units are ordered to move into a particular corner of the map, and instantly kills/removes them when they're ordered there
It's possible to remove some of the basic command card buttons so you could, for example, replace "patrol" with a "kill this unit" ability