here's Dex's finale in a nutshell:

 

"I'm betting (along with most) that Quinn planted Zach's blood under Cassie's nails. If that's the case, Quinn will expect Dex to say it's Zach's blood that was under the nail even though he already told Masuka that it was Cassie's blood, not the killer. Quinn will know that Dexter is lying, and start following him. Maybe he'll follow him to Dr. Vogel's, maybe he'll spot him with Hannah.. who knows. He's going to figure Dexter out, IMO. I don't know if he'll catch him randomly or if he'll get a hunch, but I really think Quinn will be the one to figure him out.

He's gotten a lot of face time as well, which is another reason why I'm pretty sure he's going to be very important to the final episodes."

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i'll add that quinn probably farmed the kid if it wasn't Vogel that killed te neighbor.   really that neighbor's death makes absolutely no sense.