I guess what is so hard for me is that I understand Deb's death would be extremely hard for Dexter and I get it that he would blames himself for it. However, they have developed Dexter's character as a practical guy who understands that life is full of randomness and evil that often can not be controlled or righted. Heck, that's the entire nature of the code. Life itself can't be trusted to take care of evil and so sometimes a person has to bring about their own justice. Dexter delivered that justice in his own vigilante serial killer way.
Deb's death is tragic, but for me it doesn't in any way measure up to Rita's death. Deb herself told Dexter that she got shot because the Marshall messed everything up, not because of Dexter's choice to let the police have him. While I understand he would feel partially responsible, there is clear evidence that he was not directly responsible and that overall the situation was the result of an unfortunate set of connected circumstances. It's basic correlation with an unforeseen extraneous variable as opposed to direct causation. Rita's death on the other hand is directly the result of Dexter's choice to get involved with the Trinity Killer and, in essences, start an all out serial killer cage death match. Dexter purposefully brought the Trinity Killer into his life and exposed his family to the fallout. That's why Rita's death was such a powerful and critical part of Dexter's life, not to mention one of the greatest TV season finales of all time. He was able to work through that horrific event and move past the self blame without giving up on Harrison or exiling himself for the "good of those he loves".
I just find it very dissatisfying that he could overcome one, but not the other. He worked through his responsibility in the Rita situation by recognizing that he needed to be there for Harrison. And he did all of this with no end in sight when it comes to the dark passenger. He was literally lost and didn't know what steps to take, but he knew he needed to focus on his son and moving forward with his life. Yet, when it came to Deb the writer's made him choose the exact opposite. Instead of moving onward with life and doing so with those he most cares about he decided to remove himself and basically "check out" of the game of life entirely. It just feels so empty, inconsistent, and illogical to me. Dexter has never been one who wallows in self pity or refuses to take action.