I've never really thought of this, but his deteriorating health could be a factor in making such a poor decision.
One can wonder whether Cesare's career would be better off had he remained a Cardinal. After all, despite all dangers of the Roman politics, cardinalate was much safer and more stable position than Condotierro-Prince (for example, our friend della Rovere did not lose his position within the College thorough the entire Borgia's rule). Cesare might not have become the third Borgia Pope, as his father intended, but he'd remain a powerful figure.
In a way, yes, but it should be noted that della Rovere expelled (at least until his death) several members of the college, thus potentially putting Cesare at a disadvantage anyway. Successfully returning to his lands in France might have been a safer bet, and if he had managed to stay put and quiet (easier said than done) until Luis XII and Julius II had to go to war, he might have a last chance for gaining some power in Italy.
xD, one has to wonder how could they last centuries backstabbing and murdering each other...
Are Maurice Druon's novels the ones about the end of the Capets and the rise of the house of Valois? (I haven't read them yet)