I agree. Too often these types of situations are portrayed in this manner where the situation is absolutely non-ambiguous and someone has to go get all preachy with the dialog in the story, show, etc. Knowing who that guy is, what he would have done and the fact he tried to kill you, there should be no remorse.All I'm saying is a monster is a monster. Killing said monsters will not and should not weigh on anyone. Unless we are harming/killing non-combatants/innocents, it should never come into play. None of what we've done so far should cause any moral issues.
On top of that, the kick (according to the dialog) probably crushed that guys windpipe and he was going to die anyway. At this point it could be argued you are being merciful by finishing him off.
Of course all this ignores the whole ZOMBIE APOCALYPSE! You have long ago left behind comfortable modern society. Writers need to take this into account when constructing dialog. You need to write within the world you have built.