Criminal Minds had seven main characters when it debuted in 2005. Over the years, the show went through several cast changes, leaving Gubler's Reid and A.J. Cook's "JJ" Jareau as the only two original players remaining in the end.

Reid, having an eidetic memory, makes him one of the most important assets in the Behavioral Analysis Unit (BAU).

But he isn't without his own personal struggles: at one point, he was crippled with the thought that, like his mother, he has paranoid schizophrenia.

Criminal Minds never confirmed if Reid has paranoid schizophrenia, and despite various suggestions that he is on the autistic spectrum, this was never explicitly revealed.

Despite this, there has been an ongoing dark theory that between Ried's traumatic past and implied mental state, he has the tendency to become an unsub himself.

The Paramount+ revival of Criminal Minds can take advantage of this idea by using Reid as the eventual suspect of a long-drawn case. 
Throughout their years with the BAU, Reid's co-workers have helped him tremendously with navigating some of the most difficult moments in his life. If he's revealed to have left the bureau, that means that he would have also lost the closest thing he's ever had to a family.

Being alone with no support could lead him down a dark path resulting in him becoming a serial criminal.