Why has #3 become so angry at #8?

8th Juror calls him Juror #3 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.