Certain methods and weapons are traditionally considered to be "mala in se", i.e. evil in themselves. Examples are mass rape campaigns and land mines. This article examines different interpretations of the principle that belligerents ought not to use such means. Some interpretations are reductionist in the sense that they see the principle as an instance of other principles regulating conduct in war (jus in bello), namely the principles of discrimination and proportionality. The author suggests a horizontal and a vertical dimension of the latter. Resort to violence can then be unjustified if (1) the persons are not liable to be attacked because they bear no (or not enough) responsibility for the relevant threat, (2) the amount of harm is disproportionate compared to what can be achieved by the resort to violent force, or (3) the kind of harm is disproportionate by making individual persons suffer in ways that no one should have to endure. The author defends the vertical dimension of proportionality as a key to understanding the principle of mala in se and consider whether it leads to an absolute prohibition against such means.