Megrahi was tried and convicted by a Scottish court in 2001 for his role in the Pam Am 103 bombing, which killed 259 people in 1988. After serving less than nine years of his sentence, he was released to his Libya by a Scottish court on the "compassionate" grounds that his death from cancer was apparently imminent. Megrahi was given a hero's welcome by Qaddafi upon his return to Libya and has lived far longer than the doctors cited by the Scottish court predicted.
The release was highly controversial at the time and was opposed by the Obama Administration. The fact that Megrahi is still alive has kept the controversy alive, and led to condemnations from inside the United Kingdom as well as across the world.
Romney's statement is bound to shift some of the discussion around the Libya uprising from the end of Qaddafi's rule to what happens now that the dictator appears to no longer be running the country.