reading about the politics surrounding the fourth crusade. It seems like a fatal mistake to not have disfigured the future Alexius IV in some way as to make him ineligible to the throne. Alexius III can't have had any moral reason for refraining as he had already blinded his brother.
Would it have mattered if Alexius IV was blinded? would it have stopped him from seeking help from the Crusaders? I thought physical disfigurement forbade one from becoming Emperor, but perhaps I'm over emphasizing it's importance.
Choniates doesn't explain this in his account of these goings-on. My suspicion is that Alexios III was keeping Young Alexios (Alexakos?) around as a back-up heir, in case his sons-in-laws didn't work out. Alexios III had no sons of his own, only daughters, and he seems to have been planning to leave the empire to one of his sons-in-law.
He may also have kept Young Alexios alive and unmutilated to stop Isaakios II from stirring up trouble, or as a bargaining chip with the Holy Roman Empire (Young Alexios' sister Irene was the wife of Philipp of Swabia). But I think my back-up heir theory is more likely.