four reasons can you think of?
He really did nothing, except be ignorant...

(is XVI the one, with Marie Antionette as his wife?)

Yeah, that's the one.

And the bottom answer to your question is : nothing really, other than the blind nationalism of some frenchmen who felt he had abandoned them (which he had done).

Contrary to popular belief over there the french revolution is little to be proud off - it accomplished little at a terrible cost.

Other than being French? ;)

Legally and Morally I dunno if there ever really was a real reason.

he paid for the mistakes of several generations of mistakes, he was the one handy to blame and punish

The French Revolution was good in the very first years from 1789 to 1792. I think things would have been better if La Fayette had taken the lead of the movement. However, the liberals who took the lead in the beginning didn't succeed to prevail among those who were fiercely resisting to the change. some mad extremists like Robespierre took the lead with as only political goal to restore order, no matter what would be the cost. That has severly harmed the Revolution. However, people are also exagerating the human cost in that period... and I would say that overall, it hadn't been the success expect after the period from 1789 to 1792 which was a wonderful success.

Why Louis XVI has been killed ? Well mainly because of the Nobles who didn't accept the abolishment of their priviledges and who wanted to restore the Old Regime. Louis XVI hasn't been the only one to get killed, all the family had been executed ; and if that had been the case, it was mainly to avoid the royal family to go back to power. It's been harsh and certainly extremist, but it had been felt as necessary, especially in a context where all Europe was financing the royalists. (I know there wasn't any need to mention that fact, but I couldn't resist) ;)

However, whatever we can say about the French Revolution being a mistake, we should never forget that France had then the most absolutist regime in Europe after Russia. Such a despotic rule was totally anachronic compared to the evolution of European societies, where individualism tended to prevail more and more. So before saying that revolution was a mistake and that everything was fine before, we should never forget that such a rupture was unavoidable simply because nobles weren't ready to work for a living... it was considered as morally fair for them to live with tax money and to spend their life devote their lives for superior activities than mere business.

There wasn't any Magna Carta in France (and ironically the Magna Carta had been created in France, well anyway). I hardly see how we could get rid of such a system of apartheid without a strong rupture. Overall, I think it had been good for France... certainly France wouldn't have developped more than Spain during the 19th century without the Revolution. Moreover, it has promoted a lot the idea of democracy in Europe.

Hear Hear Marla!

Not only did the revolution put an end to the Monarchy, but it paved the way for Napoleon. Napoleon institutionalized many of the revolution's reforms and inadvertrently (much to his chagrin) laid the ground work for nationalism and reform all across Europe.

Louis and Marie were in the wrong place at the wrong time. France did not have the institutional underpinnings to effect a quiet change from Monarchy to Republic. The King had to go. The response of the other crowned heads of Europe only fed the flames. Vive Napoleon!

Marla - I'll grant you that 1789 (if you exclude the July 14th atrocities that were later "written by the victors" (as they say) into an epic triumph against tyranny) to 1792 were a good start.

However, I'll note that from 1789 to 1792, France was not a full republic, but rather more of a constitutional monarchy, with Louis still on the throne and still holding a measure of power (although, of course, diminished). He was only condemened and executed in 93.