"While money was cited as a partial motive in some of the cases, Keyes was driven to kill based on a pathology that Feldis described as something "you and I will never really understand.""

it was "don't tell or i'll kill you".  

don't tell or i'll kill you is a sex rule (abuse). in one of the FBI interview videos, immediately following an instance where he had told (in regard to the NY victim, if i remember correctly) he called a bathroom break. he can also be seen in that video reaching down to his crotch area.  

but it's more complex than that. there were other things happening at the same time. he was gravely sick. suffering sick. he wanted to die to escape his suffering. in one of the videos, he was talking about dismembering one of the bodies. he gagged, and he paused. he was caught because he wanted to be. he knew he couldn't continue.

another motive was his desire to toy with law enforcement (smarts).

"don't tell or i'll kill you" made its final play when he killed himself.