In the Sermon on the Mount , Jesus recall the commandment , " You shall not kill " and then add to it the proscriptions against anger , hatred and vengeance . Going further , Christ ask his disciples to love their enemies . The Catechism assert that " it is legitimate to insist on respect for one 's own right to life . " Kreeft say , " self @-@ defense is legitimate for the same reason suicide is not : because one 's own life is a gift from God , a treasure we are responsible for preserving and defending . " The Catechism teach that " someone who defend his life is not guilty of murder even if he is forced to deal his aggressor a lethal blow . " Legitimate defense can be not only a right but a grave duty for one who is responsible for the lives of others . The defense of the common good require that an unjust aggressor be rendered unable to cause harm . For this reason , those who legitimately hold authority also have the right to use arms to repel aggressors against the civil community entrusted to their responsibility .