The truck driver wasn't a deranged would-be killer
Case in point: the school bus. Mann was too busy being concerned and warning the driver and children about the truck driver and then driving off in a hurry, that I'm sure even he was surprised when the truck driver actually HELPED the school bus get started. It's very likely that the truck driver was a hero to those kids. And Mann thought the truck driver was going to try to kill them all.
It makes you wonder if Mann was a hero for destroying the truck at the end, or if he had just tricked an innocent man into crashing to death.