Dead Island: Riptide Definitive Edition

There's a few ways to go about this. Most importantly to note, is only the finishing blow needs
to be a kick. So you can wittle down their health with weapons and then kick them with .
It does more damage when they are on the floor.

Alternate strategy: Requires imported character from Dead Island (with stomp ability)

However the fastest way I find is to jump kick a zombie ( + ) and then whilst they are on the floor, another prompt will appear on the head. Simply press once more for an instant kill. Note that this won't work on special infected (the powerful types) apart from the fast running zombie which is simply called 'Infected'.