I truly don't know, Nyongessa.  I am answering here to acknowledge the question more than offering an answer. Mary of Scotland was executed to end a challenge to the Crown, Elizabeth, perhaps, but the other examples don't fit this reason.

Hitler saw no future in defending himself or the ambition of the Third Reich. He used cyanide and a pistol, taking no chances. Dealing with Bin Laden, Houssein, and Quaddafi as captives or prisoners was clearly more trouble than it was worth. At least Saddam got a day in court sort of, if only a Kangaroo court.
But I don't know that there was any sort of warrant out for Quaddafi's death and maybe it was an accident but I doubt it. What I take away from it is that there is no longer any real need for legal proceedings. The right to die may be questionable, but killing is not really problematic. We are used to it. Nobody really complains, certainly not the dead. 

I'm not an expert on the history of the Middle East / Near East / Maghreb, however, one can easily seem to be one nowadays thanks to the Internet. I leave it for you to judge.  What I do know  is that the 'Arab Spring' is changing the face of Arab history but there is no certainty as to what lies in store for the wider world. Tunisians deposed their dictator, Ben Ali, and will vote in free elections tomorrow. That's good news and let's keep our fingers crossed for them. Libyans just got rid of their megalomaniac dictator Gaddafi, with a little help from NATO. Those who live by the sword, die by the sword. Another tyrant down - so who's next? Syria, Iran ?

It is certain that the Arab world is short of the basic freedoms we in the West take very much for granted. They are mainly ruled by brutal despots who refuse make way for genuine democracies. Their people deserve better and time is on their side, but I wouldn't say that progress per se necessarily means that the 'end of history' is just around the corner. There are many geopolitical, ecological and public health dangers facing this planet of ours at this present moment.  Cautious optimism is better than cynical pessimism.