Technically she wasn't dead. Brain dead with the hope of one day making some degree of recovery.
Masuka getting to know his daughter for 35 seconds to a minute per episode had nothing to do with Saxon and really didn't do much to the season; much like Quinn losing to the no name detective or Batista searching through LaGuerta's shyt. All filler scenes with no impact on the overall arc. And let's be real, Saxon will always be viewed as lesser than the greats, but he represented the polar opposite of Dexter this season and
while his presence didn't dominate like ITK, Prado, Trinity, Lila, Doakes or even Lundy, he played his part and did it well in the time he had.
Ultimately this season was all about the Dexter/Deb relationship - everything else was filler to get to that point. And in that, they succeeded. And you're correct, he wasn't about impulse in the scene with Saxon, but across the board Dexter has been consumed by his need to kill. Finally fighting that need and NOT killing him came back to bite him in the ass and kill the most important person in his life. But in general, he needs to know. He needs to be in control. It's why he couldn't leave Miami without knowing Saxon was taken care of. It's why, even when in custody with the police, he went for his get back. He needed to see Deb, one last time, his way. If she was going to die, it would be with him, no one else. Not while he's broken and in the wilderness as a recluse. He felt responsible for her death and did things the way he knows how. It was very fitting and more importantly very well done. It's been hours and I still feel it in my chest like I lost a family member watching this.
I can't break it down any further for any fans that don't see it 
She couldn't breathe or anything on her own. Dexter pressed the button and let her die of natural causes (her body couldn't keep her alive).
I won't get into how preposterous it was that that he did it in the middle of the ICU, in a Miami hospital where the doctors and nurses are as lazy/dumb as Miami Metro. Go walk into the nearest hospital and turn off someone's breathing machine and see if any doctors or nurses come sprinting into the room.
Then he walked right out with a patient and loaded the patient onto a boat IN FRONT OF EVERYONE. And LOL @ the trees being blown sideways when he walked into the hospital, yet a fukking white sheet remains undisturbed whilst draped over Deb.
Here's another one...
Deb's body disappears, Dexter's body disappears, and they publish a freaking "We'll miss you Dexter!" post in the Miami news?
I know that Miami Metro is composed of the biggest morons on the planet, but your local elementary school hall monitors could solve that mystery.
I can see it now:
Quinn: "They said Deb's body was gone. Along with the white sheet she was tucked in with. Just like Jesus."
[Everyone looks around at one another, nodding]
[Camera pans out so audience sees cast standing around Deb's bench, just 10 feet away from Laguerta's]
Not "Brother and sister go missing during Hurricane Moron" (or whatever the fukk it was called)
Speaking of the hurricane, the only thing that the hurricane was symbolic of was just how big of a shyt storm this show had become.
RE: The Saxon character and presence. The actor may have held his own, but Saxon's character in general was ridiculously horrible when compared to every other adversary in the history of Dexter. His introduction, the buildup to what his character would become, the way that his character was outed (computer scene), his signature... uhh... well there wasn't really anything unique about him.
RE: Dexter/Deb: In other seasons, the writers were very skilled in taking an issue like Dex/Deb and lacing it in and out of other developing and ongoing plots of the show. This season they violently deflated the Deb/Dex relationship like a soccer ball getting stabbed to death by menacing twelve year olds in a Rio alley.
RE Your last sentence: I get it, you held strong for the show. I respect your loyalty and dedication. Myself, and many many other fans of the show, can't get past how blatantly stupid the writers have played the viewer for the past 2-3 seasons. I can't break it down for you either.
All of that said, I'm still grateful that I got to see the first four seasons of Dexter play out the way that they did. Ultimately, they were never able to replace the characters that they killed off in the first four seasons. When Rita was dead in the tub and Harrison was crying in the pool of blood on the floor the series as a whole sealed a tomb on something that could never be recreated or maintained. It was strong enough to keep most of us on the line through tonight.
. I never want to be buried at sea, damn. I think the episode was good even though the plot direction was a bad choice. Part of me feels like this as a final season is 
and next summer a new season is just gonna pop out of nowhere. Honestly with all the bad reviews it may be possible to salvage a decent ending.
