I don't think Petain was a fan of the Nazis.

He was a reactionary by inclination -- few who come to power in the mid 80's are noted for flexibility in thought -- but I think his status as a "traitor" was as much for being a traitor to De Gaulle as to France.

H. P. Petain was always a bit of an "odd man out" among French War leaders. Prior to WWI, he preached defensive tactics and firepower to an army enchanted with the doctrine of "offensive a outrance." De Castlenau put his army into the breach at Verdun - and made Petain a national hero - because he was one of the few defensive generals who had a good grasp of artillery. Petain was then booted and command vested in Neville because they wanted someone who was aggressive. Petain was never particularly aggressive, and was considered very "in touch" with the common soldier throughout his career.

In 1940, he came to the pinnacle at a bad moment for France. French troop strength was signficant but their combat doctrine was outdated, their operational formations jumbled, and their morale weakened by the pace and disorienting effect of the German blitzkrieg. Petain did not believe that the French army wouldn't fight, he had come to believe that it could not fight effectively -- and like many veterans of WWI, he had no doubt come to abhor the idea of killing so many soldiers when he did not believe he could change the eventual outcome. Was he right?

Though reactionary enough to allow anti-jewish measures in Vichy, Petain was never a fan of the Nazis and tried to be a "neutral." The Laval affair demonstrates that clearly. Petain wanted Vichy to be the kernel from which France re-grew itself. I suspect that he hoped for Germany's defeat, while believing that France was out of the fight.

Why not declare war after Mers-el-Kabir? I suspect it is because he did not want to become a Nazi ally -- which he would have become in fact if not in name (it was bad enough being a puppet) -- because then France would have been fully split and a civil war (at least after Germany's occupation ended) would have become almost inevitable (there was certainly enough tension as it was with Vichy, De Gaulle's Free French, and the often communist-led members of the Maquis). He may have hated the English actions, but the "honorable" alternative would have led Vichy down and even darker path.