John Freeman, you let next boss step on me and made me headcrab zombie! Combines came and put science in me and made me live and strong and big now! I make you and Henry Freeman headcrab zombie! Prepair to die!
Actually it makes perfect sense, you see, obviously not even bullets could express the rage John Freeman accumulated after he heard about the tragic loose of his wife, killed by hands of the Combine, who he loved like sun raise. And as the enemies around him swarmed, John Freeman realized he couldn't kill them all with his trusty wepon, therefore venting his uncompromising anger on the attacking Combine forces by his bear hands.