Almost all the Buddhist teachers I can think of (including me too, for what it is worth) would say that we must also bear all the Karmic consequences of our volitional words, thoughts and acts, no matter whether we had a "reason" for killing or not.
You may kill the cat, but you still likely have to pay the price in some way.
A Tibetan teacher (Chagdud Tulku) relates this famous Jataka legend about a previous incarnation of the Buddha ...
(In a previous life, the Buddha was Captain Compassionate Heart, sailing with 500 merchants. An evil pirate, Dung Thungchen (Blackspear) appeared, threatening to kill them all. )The captain, a bodhisattva himself, saw the [pirate]'s murderous intention and realized this crime would result in eons of torment for the murderer. In his compassion, the captain was willing to take hellish torment upon himself by killing the man to prevent karmic suffering that would be infinity greater than the suffering of the murdered victims. The captain's compassion was impartial; his motivation was utterly selfless.
I am not sure about the effect of our Karma in lives to come ... but I do know that we likely will bear the effects of our actions in this life in some way. I have a friend, an ex-policeman, who had to kill someone in a perfectly necessary and justified act to save lives. Yet, my friend still carries that with him to this day.
No, taking lives is never a "good" thing.