I think in your conclusion there is a great lack of consideration for nuance. Do you really think that the likelihood that a given baby will grow up to become the next dictator is equal to the likelihood that a maniac and chaosmonger, who literally lives to sow destruction, will strike again?
I have to say, both of your answers (granted, Amadeus hasn't answered yet, but he comes and goes like the Dark Knight himself!) surprised me. Neither of you addressed what to me was the biggest debate about the situation: Batman compromising his own principles in pursuit of a greater goal. Batman desires to rid Gotham of the destructive criminality and gangs which terrorize the city and obstruct its prosperity (I'm purposefully ignoring Ra's Al Ghul right now), etc etc. But he also is firmly against killing. With the regular gangsters and the like, the people who play by rules, but weird rules which don't correspond to those of the rest of society, the situation can be solved by wrecking their organization, and a little bit of vigilantism in tying their leaders to spotlights. The corruption in the police force can be fought by tracking down the cops using someone from outside the force, which will enable them to become more effective as they become a tighter-knit functional crime-fighting body. But the Joker? He can't be turned over to the police. He can't be prosecuted, locked away for a few years, and then released into a world more capable of handling his corruptive intentions. The only way to "beat" the Joker is to do the one thing Batman won't do: drop him off that building. =
This was why I really asked that question: would you ever infract upon a principle you hold dear in order to forward another principle you hold dear? Would you kill selectively, but avoid it otherwise if at all possible, in order to forward the cause of peace or justice?