I finally came to the conclusion that he was just angry and frustrated and didn't know where to go, what to do or how to achieve it, because there are numerous opportunities for him to advance himself that he doesn't take, despite your assertion that he is driven by ambition. The essential problem he had was that he had no patience or planning or forethought, so he passes up long-term opportunity for instant gratification.

I often wondered if Stendhal was engaging in a subtle critique of the notion that the low-born could amount to something. After all, Sorel's behavior would make for several great episodes of trailer-trash daytime television (à la Jerry Springer). An affair with a married woman, a dubious ecclesiastical background, attempted murder...

Essentially, I think that Stendhal can be forgiven for having a main character who doesn't behave in ways that always make sense to the reader, because his novel is one of the first "psychological" novels of the Nineteenth Century, and he certainly influenced Dostoevsky, who expanded on (and, in my opinion, mastered) the trend. The book is a bit uneven because it's an experimental book for its time.