It's not until Juror #3 starts talking about his own son that we see the true reason for his bias against the defendant. While thinking about the trial, #3 talks about how he once saw his own son run away from a fight. As #3 tells it, "I told him [my son] right out, 'I'm gonna make a man out of you or I'm gonna bust you up into little pieces trying.' When he was fifteen he hit me in the face. He's big, you know. I haven't seen him in three years. Rotten kid!" So it's clear now that this guy has a bit of a problem with kids not respecting their fathers.