"the moral principle of choosing the lesser of two evils"

As has already been discussed here, I suppose that depends on what you consider the "two evils" to actually be? Bin Laden is already being described as "Obi Wan Bin Laden" online for example, or in other words, more powerful dead than alive.

Here are some considered comments on the possible long term fallout of this incident and the dangerous potential consequences of a resultant "redirection of the Arab spring":

Ed Husain, Senior Fellow at the Council on Foreign Relations, says that Osama bin Laden is more valuable to al-Qaeda dead than alive.