I do not suggest it is a decision easily made, or made lightly, however I think the slaughter of innocent civilians justifies intervention. Now I am not sure why you say the rebels are of "dubious pedigree", and while you may have doubt about them, I have certainty about the dangerousness and internationally destabilizing bad acts of the colonel. He has been a blight on Africa and the world for a long time.
From a legal perspective the potential for genocide gives us the right to intervene.
Here is also a realpolitik analysis of why we should intervene that makes sense to me:
http://www.cnn.com/2011/OPINION/03/07/frum.libya.choice/index.html?iref=allsearch
The dubious pettigree is a concern about the origins of the "rebels" . Are they citizens revolting against the government, or are the Taliban/ or other groups of similar ideals seizing an opportunity to get a legal government cover for their operations. Do we know who they actually are? I don't and a number of other friends are unable to identify the leaders of the rebel groups whichis a concern.
I have a problem with the phrase genocide, it is being overused , genocide would refer to the killing of all of an indentifiable group, ie christians in a muslim country or a specific nationality, in this case I do not see a targeted identifiable group being systematically hunted down and disposed of. The possibility of large scale killings is entirly possible, upto the point of mass murder.
As far as the colonal is concerned he is in my opinion, barking mad and should have had his story ended years ago, Ronald Regan missed him in the 80's which was unfortunate , however part of the problem I am refering to what happens when/if he is replaced does a Taliban mulah take over? As bad as Gahdaffi is there are worse options as Afghanistan following the Russian withdrawal showed.
The Frum article is interesting and from a regaining American profile in Africa and the middle east it does make some sense, the problem (IMO) anyway is that it will cost the americans more stature if the "wrong" people get control of Libya which is a definate possibility . The American record of supporting the people they put in power is questionable at best , from the SHah to the Kurds and a number of others the only country with non stop support has been Isreal and that is almost entirly due to the large Jewish population in teh states.
It is a complex situation and with the domestic problems facing both American and British governments are their constituants going to allow it to happen, ( intervening) . France has been making noises in that direction but they will not do anything without somebody else to do the work and the Germans will not allow their armed forces to go invading .
It all comes back to do we have the right to intevene in a civil war? If we decide we do what happens if China decide to invade Taiwan ? The consider it a province do we intervene in that one?