what is #8 point about the "I'm going to kill you" line

8th Juror calls him a “self-appointed public avenger” and a “sadist,” and 3rd leaps at him. Restrained by the other men, he shouts, “God damn it! I’ll kill him! I’ll kill him.” 8th Juror asks, “You don’t really mean you’ll kill me, do you?” proving his earlier point about how people say, “I’ll kill you,” when they don’t really mean it.