In the play 12 Angry Men, why does the usually calm 8th juror become angry?

I think this might be when the 3rd Juror reacts violently and ends up attacking 8th Juror, shouting, “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.