How was it off camera? She died when Dexter pulled the plug on her. And I don't think it was handled casually at all. It was the core of the entire finale episode.
Anyways, something I find interesting now that everything is said and done with this show is that there's a lot of openings to skip a lot of the bad in the later seasons and still have a fairly decent ending.
You could stop at the end of Season 4 or at the end of the first episode in Season 5 so you get a conclusion to Rita's death. You could stop right before Deb shoots LaGuerta and instead choose to believe she shot Dexter. Worst of all, you could literally go from Season 7 to the final 2 episodes of the show and it'd probably be even better, which is what someone on reddit did.
Spoilered for length:

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