To be fair to Valls, he didn't betray Hollande. On the contrary, he refused to announce anything before Hollande himself decided to stay out of the race, and only then (and after meeting him) did Valls take his decision.
For all the criticism that he gets, I still have some respect for him, because he does have some sort of integrity, except that it's more towards his beliefs rather than his party (which is how it should work anyway). What I dislike about him is that some of those strong opinions he has on specific social matters, where he can emit surprisingly Sarkozyst vibes, but apart from that he's not a bad politician.