You have to want to be a better fighter in the game in order for taking more levels in the class to make sense, you must pray and be faithful to gain higher understanding as a cleric, if you want as a warlock to not go futher down the dark path you just don't.
As a DM you could say that a warlock has no choice but to take further levels of warlock because of the pact he made, but then that takes away player choice wich is something 3E and apparently 4E try to never ever do. The game has become about choices, not railroading PC's down a career path.
So this whole tragic character concept is just an excuse to play an "evil" character but at the same time going he is just misunderstood and dark. When it is a choice he makes every time he levels to go futher into the darkside, he could go to a temple and start taking levels of cleric anytime he has the chance, the player knows it his character knows it and the DM knows it.