Jamieson, Fausset and Brown on James 1:15

15. The guilty union is committed by the will embracing the temptress. "Lust," the harlot, then, "brings forth sin," namely, of that kind to which the temptation inclines. Then the particular sin (so the Greek implies), "when it is completed, brings forth death," with which it was all along pregnant [Alford]. This "death" stands in striking contrast to the "crown of life" (Jas 1:12) which "patience" or endurance ends in, when it has its "perfect work" (Jas 1:4). He who will fight Satan with Satan's own weapons, must not wonder if he finds himself overmatched. Nip sin in the bud of lust.