Evil shall slay the wicked may be a way of saying that the wicked person’s own evil plans will eventually destroy him (so Biblia Dios Habla Hoy, Bible en français courant, New English Bible); or else it may be a reference to divine punishment, which is the thought of line b: they will be condemned, by God, that is. The Hebrew verb translated condemned (Good News Translation “punished”) occurs very few times and seems to mean “to be held guilty”; the idea may be expressed by “brought to ruin” (New English Bible), “be ruined” (New Jerusalem Bible). New Jerusalem Bible translates “will pay the penalty.” Because in some languages it is not possible to speak of an abstract quality such as evil being the agent of the act of killing, it will be necessary sometimes to translate “The evil person will cause his own death” or, if understood in the sense of God causing his death because of his evil, one may sometimes render it “Because the wicked person is evil, God will destroy him.”

Quoted with permission from Bratcher, Robert G. and Reyburn, William D. A Handbook on the Book of Psalms. (UBS Helps for Translators). New York: UBS, 1991. For this and other handbooks for translators see here .