The following is Clyde Phillips, the Show Runner from the first 4 seasons; this is his diea of how he would've end Dexter;
"In the very last scene of the series," Philips explained, "Dexter wakes up. And everybody is going to think, 'Oh, it was a dream.' And then the camera pulls back and back and back and then we realize, 'No, it's not a dream.' Dexter's opening his eyes and he's on the execution table at the Florida Penitentiary. They're just starting to administer the drugs and he looks out through the window to the observation gallery."And in the gallery are all the people that Dexter killed—including the Trinity Killer and the Ice Truck Killer (his brother Rudy), LaGuerta who he was responsible killing, Doakes who he's arguably responsible for, Rita, who he's arguably responsible for, Lila. All the big deaths, and also whoever the weekly episodic kills were. They are all there.
"That's what I envisioned for the ending of Dexter. That everything we've seen over the past eight seasons has happened in the several seconds from the time they start Dexter's execution to the time they finish the execution and he dies. Literally, his life flashed before his eyes as he was about to die. I think it would have been a great, epic, very satisfying conclusion."