I usually stop watching Dexter around Season 4, Episode 7.
Rant
Dexter not killing Trinity feels like a plot contrivance; there’s no real reason for him to let him live. He just decides not to kill him after already receiving the proof he needed. Instead, he gets sidetracked with some serial killer of the week because “he threatened Deb.” Really, Dexter? Do you not remember what happened just a few episodes ago because of Trinity?
The idea that “Trinity is on cooldown” doesn’t make sense. When has that ever been a consideration for him? It felt like the showrunners didn't want a predictable series, so they made Dexter act out of character. Trinity isn’t just another Miguel situation; Dexter has seen him kill innocents live. There’s no justification for not dealing with him ASAP.
You could argue that Dexter is struggling with parenthood and being a husband, which clouds his judgment. However, he has never allowed his role as a boyfriend and pseudo-father to so severely distort his logical reasoning. Show runners wanted Trinity to hurt Dexter. The only way they could get that outcome was to make Dexter act outside of his norm.
“Maybe he wanted to learn more about having a normal family life from Trinity.”
Dexter has tabled other serial killers with normal lives. He knows how’s to put on a show and still exist.
“Well Rita needed to go out that way for the set up!” (in a totally different show that the writers definitely knew would be a reality someday)
Actual take I’ve seen before. That show also shifts the dynamics around with the audiences relationship with Dexter so not a fan. I’ll explain. Imagine if Supernatural had a spin-off show that said: “erm actually the Winchesters were pieces of shit the entire time. You should hate them actually. Because technically they were still killing people even if they were irredeemable monsters!”
Then Sam and Dean are killed off by Adam and that found footage werewolf they chose not to hunt.
That’s how that ending feels. So no that “setup” isn’t a good payoff to either series. I’ll just pretend the show ended after S3.