2 Clearly, divine law, both that which is known by the light of reason and that which is revealed in Sacred Scripture, strictly forbids anyone, outside of public cause, to kill or wound a man unless compelled to do so in self defense. Those, moreover, who provoke a private combat or accept one when challenged, deliberately and unnecessarily intend to take a life or at least wound an adversary. Furthermore, divine law prohibits anyone from risking his life rashly, exposing himself to grave and evident danger when not constrained by duty or generous charity. In the very nature of the duel, there is plainly blind temerity and contempt for life. There can be, therefore, no obscurity or doubt in anyone's mind that those who engage in battle privately and singly take upon themselves a double guilt, that of another's destruction and the deliberate risk of their own lives. Finally, there is hardly any pestilence more deadly to the discipline of civil society and perversive to the just order of the state than that license be given to citizens to defend their own rights privately and singly and avenge their honor which they believe has been violated.
Source: Pastoralis Officii (Vatican.va)