There's nothing they could have done with Rita instead. Killing her was the shot in the arm this show needed. S3 demonstrated that there was a big problem developing in the show - that without some huge smack-to-the-head for Dexter, the dramatic oomph was starting to sputter away.
It isn't good drama or interesting for us to watch Dexter just putter along at the same level as he was at, during S3 - learning to become a normal human being in comparison with the darker folks around him. Dramatic tension should rise as a story progresses, and in S3, it was deflating like a balloon. It wasn't a bad season, but it was smart of the writers to circumvent the problem before it became a particularly bad problem. Another season like S4 and it would have started to be a bad problem.
Now they've amped the drama by giving Dexter what could be an insurmountable challenge - to continue the path he was on regardless, rather than spiraling into the outer darkness. They could have done the same thing by killing one of the kids, but that's too dark for even this show. They could have killed Deb, but she's too important to the plotline. Far more important than Rita, since Deb represents the last solid link Dexter has to Harry; and as a cop, Deb represents yin to Dexter's yang. Rita may be good for Dexter's morale but Deb is literally and metaphorically central to the issues of morality in Dexter's story, and therefore central to the whole story.
Nobody else is emotionally important enough to have given the show what it needed by dying. It was obvious to me earlier in the season that Rita had to die, and I hadn't been reading spoilers at all. It was apparent because I recognized the danger the show was getting into, and what the cure had to be.
The fact that the writers recognized that they were going to run into a problem, and recognized exactly the best way to nip it in the bud, gives me more respect and confidence in their abilities. Compare that to a show like Heroes, where the writers/producers seem only dimly aware of what their problems are, and are utterly clueless how to solve them. If only all shows were as deftly and pro-actively written as Dexter.
