The Catholic Church teaches that we cannot do evil so that good may come out of it. It cannot be the means as in abortion. It could only be tolerated as an undesirable consequence of a good act such as self-defense. But unless one is in immediate danger from the agressor, I do not see how one can use deadly force. Whether the partisan action is legitimate self-defense I cannot say, I didn't actually look into this, but the disposition of treating whst you believe is a sin as a good essentially, and hoping for forgiveness without repentance, that is evil.