Why didn't Charnier kill Doyle?
After kidnapping him and feeding him heroin, why did he give him back to the police? Barthelemy says something to Doyle like, "Imagine if your police force finds out you're a junkie cop" to explain why Doyle's rehab had to be kept secret, but if Charnier's reason for doping him up was just to make him look like a junkie, that seems kind of dumb. And if Charnier considered Doyle to be such a fly in the ointment, it seems that it would make even more sense to just kill him.
Any theories?