By "irrational", do you mean in the sense of "would pay the $100 as Parfit's Hitchhiker"? If so, then the answer to all three questions is yes: there are lots of scenarios in real life where we are called upon to pay debts both positive and negative (repay favors, retaliate against aggression) and we think the benefit to be gained from doing so will be less than the cost. There are enough such scenarios that a disposition to pay debts without stopping to do utility calculations usually pays off handsomely over a lifetime.