Bruno,

that's very interesting, and I agree that Napoleon inner political choices, although bold, fit to the situation.

But I would put more personal ambition on him :

We can consider that, by seizing power as first consul, Bonaparte deliberatedly chose to be on, not behind the throne. Even more since, when Louis XVIII proposed him to reinstate monarchy, he bluntly refused saying he will be "no Monck".

For the decision of becoming emperor, I don't see who or what could force him to choose this way, which was both not so tempting for his ex-republican core partisans, and much in line with his personal view of strengthening both his power and the french nation by creating coherent institutions.

So no megalomaniac whim, only sensible political choices.

(it seems I will use sensible a lot of time, now that I discovered it )