As a tiefling, you're born as a ticking bomb. Your ancestors wrote off your soul into the service of the Nine Hells. Some tieflings embrace the notion, becoming true villains. Others try to redeem themselves through pious behaviour.
A few, more pragmatic, decide to make a pact with entities beyond even the scope of the devils. Sure, it is a desperate gamble, but who knows? Perhaps if you amass enough power, you might be able to resist not only against the Nine Hells, but also against the Far Realm.