This may be the first ever time outside of videogames and bad printed fanfic where the viewpoint detective is shown to be a perp.
Well, closest we got is this one Christie Novel where a POV character who aiding poirot IS the perpetrator.
Impossible...Someone's gone and eaten up all my potatoes...!Terrible news!SHIBAFUU!A case, yes!The culprit lies within...SHIBAFUUUU!All the passengers are culprits...!?The Great Detective Warspite, who just happened to be riding along with you,*empty*Shall solve this case splendidly!