Some of the "NPC's" that you can't talk to have finite hp. You can't shoot them, you can't melee them, but you can shove them to death. The guy at the fence with the gun in the first town and the army guys in front of the stadium, I know you can. I'm pretty sure you can't do this with the guys at the end, but for some reason, they still have hit detection for shoves.
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