55. This should not cause surprise: to kill a human being, in whom the image of God is present, is a particularly serious sin. Only God is the master of life! Yet from the beginning, faced with the many and often tragic cases which occur in the life of individuals and society, Christian reflection has sought a fuller and deeper understanding of what God's commandment prohibits and prescribes. 43 There are in fact situations in which values proposed by God's Law seem to involve a genuine paradox. This happens for example in the case of legitimate defence, in which the right to protect one's own life and the duty not to harm someone else's life are difficult to reconcile in practice. Certainly, the intrinsic value of life and the duty to love oneself no less than others are the basis of a true right to self-defence. The demanding commandment of love of neighbour, set forth in the Old Testament and confirmed by Jesus, itself presupposes love of oneself as the basis of comparison: "You shall love your neighbour as yourself " (Mk 12:31). Consequently, no one can renounce the right to self-defence out of lack of love for life or for self. This can only be done in virtue of a heroic love which deepens and transfigures the love of self into a radical self-offering, according to the spirit of the Gospel Beatitudes (cf. Mt 5:38-40). The sublime example of this self-offering is the Lord Jesus himself.
Moreover, "legitimate defence can be not only a right but a grave duty for someone responsible for another's life, the common good of the family or of the State".44 Unfortunately it happens that the need to render the aggressor incapable of causing harm sometimes involves taking his life. In this case, the fatal outcome is attributable to the aggressor whose action brought it about, even though he may not be morally responsible because of a lack of the use of reason. 45