I agree with this. And I might even be persuaded that to kill in defense of ourselves or others makes the killing not our 'choice' (and therefor not our 'act'). It would be a re-action that was forced upon us by the attacker, and therefor the attacker's death would not be our doing.
BUT, this would not change the fact that killing another human being is always wrong. Because we would simply be shifting the 'act' back onto the perpetrator, and away from the respondent, even though in this case it would be the respondent who ends the life of the perpetrator.
It smells a bit of sophistry, but I suppose it's a logically valid argument. And I could concede to this reasoning.