It is a primary law of nature that every human being should and will strive to resist injury and destruction. Justice requires a moral equation, and if one right prevails over another it must be superior to the right it supersedes. At the outset both the aggressor and the intended victim have equal rights to life, but the fact that the aggressor uses his own life for the destruction of a fellow man sets the aggressor in a condition of juridic inferiority to the victim. The moral power of the aggressor here is equal to his inborn right to life, _less_ the unrighteous use he makes of it; while the moral power of the intended victim remains in its integrity, and has therefore a higher juridic value.
The right of self-defence is not annulled by the fact that the aggressor is irresponsible. The absence of knowledge saves him from moral guilt, but it does not alter the character of the act considered objectively; it is yet an unjust aggression, and in the conflict the life assailed has still a superior juridic value. In any case the right of wounding or of killing in self-defence is not based on the ill will of the aggressor, but on the illegitimate character of the aggression.
The condition's of a blameless defence (_moderamen inculpatae tutelae_) are: (1) that the aggressor really threatens the defender's life, and there is no means of offsetting that violence except like violence; (2) that no more violence is used than is adequately required: if the aggression can be stopped by wounding the aggressor the defender is not to kill him; (3) that the violence in the defence is used with the intention of defence, not in revenge, hatred, anger, or the like motives.
We may do an act good in itself from which a double effect immediately follows, one good, to which the agent has a right, and the other bad, which the agent is not obliged to omit if permitted by him and not intended; but in the case of a necessary defence of life against an unjust aggressor, made even with the death of the aggressor, the defence is such an act, provided the moderation of a blameless defence is observed.
The evil effect here is not a means to the good effect, nor does it more immediately follow from the act done. The evil effect is an effect _per accidens_, and thus not directly voluntary, either in itself, because it is not intended, or in its cause. It lacks the condition necessary to make it voluntary in cause as regards the accidental effect since the act is not prohibited precisely because this accidental effect follows.