I think it makes sense that a little demon might shit its pants and leg it when I slaughter a Baron of Hell. Especially if he's alone. And wounded. Also, the greater the contrast with the one I killed and the one running away, the bigger chance of SQUAD BROKEN. Even bigger chance if you're berserking and running after him with a chainsaw or something like that.
It would also make sense that when hit points go past 100%, you'd have enough energy to keep running indefinitely. It'd increase the decay rate, though, and when HP hits hundred, you'll have to stop.
Speaking of which, there needs to be a trait for running more. Marathon. Requires two points at Hellrunner. Doubles the time you can keep running before getting tired: Once your normal running time is out, it goes back to cautious instead of tired, and you can have another run before really burning out. (When combined with the above suggestion, it'd decrease the decay rate of HP while running.)