So, this still has to be used while the opponent is attacking, right? The name doesn't seem to fit the ability, since you're waiting until they've attacked for you to go on the defensive. A preemptive measure would be killing a creature before they even attack.

Agreed. Normally, I design cards and deal with names and flavor much later... but with Multiverse, I get no feedback unless my cards have names. So it is.

The thought behind the name was that you are killing the next creature before it gets to attack... but without flavor text, this doesn't come across... and it might not with flavor text, either.

I've been stumped for a name for this. I wanted something that represents "Total War" or "There are no such things as innocent civilians" but just can't get the right expression. Any suggestions?

Collateral Damage?

The best I was able to come up with was "Take No Prisoners" which fits... but seems too cruel for white. I like the "These aren't human beings, they're obstacles" approach to 'Collateral Damage'. It's, unfortunately, is a bit buzzwordy, but sounds better on a Magic card than "Authorization of Lethal Force".