The United States has a long history, especially in the middle east, of supporting drastic leaders who ended up becoming serious problems for us later, including Bin Laden, himself. Supporting Gaddafi in this way would only repeat the cycle. Favoring one strong-man criminal leader over another, only to find neither were a smart bet.
We can all go around saying would of, should of or could of all we want about the past, but it doesn't change what went down. Osama Bin Laden was taken down eventually and maybe some felt Gaddafi would of done it faster but no one knows this for sure. This is a done deal and the outcome is what it was.
Giving tacit or actual support to a dictator when it is convenient and expecting him to fall in line when it is not is simply a fallacy. Forming partnerships with people like Gaddafi is a sure way to sling enough rope to hang yourself. The prestige he would have gained by having U.S. support on mission like that would further undermine his grasp on reality.
US did not use Gaddafi to hunt down Bin Laden. US did the right thing by taking on Bin Laden through its efficient Navy Seals. If US allowed Gaddafi to kill Bin Laden, it would have resulted in a more chaotic situation across the borders. Gaddafi himself changed his ideologies over the decades how LIbya should be ruled.