One Lawgiver and Judge
James forbids speaking evil against a brother or sister because slander turns the speaker into a judge over the law rather than a doer under it. The argument reaches upward: there is one Lawgiver and Judge, the one able to save and destroy. “Who art thou that judgest another?” exposes the presumption beneath destructive speech.
Where this fits in the story: Torah condemns slander and false witness, wisdom literature warns against destructive speech, and Jesus warns against hypocritical judgment. James combines those streams with his earlier law-of-liberty theme.