There is a strong hardwired social expectation for a fair (and I won't even use quotes here) split, and there is an emotional cost in overriding that. You can decide it's worth it, but if you just ignore this cost, you are, by definition, not being rational. And once you start factoring any non-monetary factors, the $0.01 vs $4,999.99 Nash equilibrium breaks down.
If you go to the wikipedia page for the Ultimatum Game you'll see that across cultures, almost everyone will reject any offer below some 20%-30%. So, you see, the vast majority of humanity is very "weak" of character.
This and other experiments suggests that we are hardwired to tend towards collaboration and towards punishing defectors, because it has proven evolutionarily stable.
In a real world where almost nobody will take less than 20%, is asking for $4,999 rational?
In the extreme case you were presented before, after the Splitter has killed your family, would you still accept his $10 vs $4,990? What do you mean he "deserves" revenge? That's not any more Nash-rational. What does the killing of your family have to do with the deal, at all? Is there something wrong with your character?