"...Gaddafi's open threat to kill one million people..."
Oh, were you thinking of this?
"Gaddafi's army will kill half a million, warn Libyan rebels"
Half a million, and actually it was his enemies who said that. I think you'll agree that's slightly different. If I said you wanted to eat all the babies in England, would that justify other people in killing you to prevent it happening? If you are really interested in the facts, try starting with these short and digestible pieces:
In particular, let me commend to your attention this undisputed fact:
"In April 1986 US warplanes struck Tripoli at 2 am. They bombed the Gaddafi family residence, wounding several of his family members and killing his 15 month old daughter".
Killing innocent civilians, eh? She would have been 26 now.
"Your argument seems to suggest that there is no legitimate way to replace a dictatorship by force - I suggest you might have a different opinion if you had lived through one (or had a tiny iota of empathy)".
You're right: I do think there is no legitimate way to overthrow a government. It can only be done by violence, usually, and violence is intrinsically illegitimate. Of course you will disagree if you live in a Hollywood universe where the white hats always triumph and the black hats bite the dust. Trust me, reality isn't like that at all. Even the USA, with its cherished and inspiring Declaration of Independence, had to admit that the principle of self-determination didn't apply when citizens of the USA itself wanted to break away and set up their own nation. It crushed them ruthlessly at the cost of at least 620,000 lives.
Dictators vary dramatically. I have in fact lived under Juan Peron, Antonio de Oliveira Salazar, Marcelo Caetano, and Habib Bourguiba. My parents spent quite a lot of time in Germany in the 1930s, and told me what it was like living under the Nazi regime. So yes, I have lived under dictators - and the news is that it's a lot like living under a democracy. Only sometimes, as in the case of Salazar, there is a bit more sensible and consistent long-term planning.