I was probably just as relieved as you were when Shara when came back to life. After all, I built my original fantasy around her. She's a completely different person in this version of the story, but that doesn't make her any less important to me. I hated killing her. I just felt like I had to do it.
Here's how the decision came about.
Almost exactly a year ago, Callidus and his wife came to visit me. I hadn't yet begun to write Sleepwalkers, but I was pretty far along in the brainstorming. My main dilemma, at that point, was figuring out what to do with Shara. I knew that Paul would be wracked by guilt because he'd imprinted her, and that when Shara Awoke she'd forgive him; but it couldn't be that easy. Callidus was the first to realize Shara had to feel guilt of her own. Justice is her driving force, and she's done some horribly unjust things (not least to Wizard). Now she'll have a chance to try and make them right.
I probably already knew, when I met Callidus in person, that Shara would guess Paul was Awake; but I didn't know how she would confront him. That came much later. I bounced a lot of ideas off Callidus via email, and gradually I homed in on the idea of her testing Paul's loyalty by making him kill her. It was such a shocking idea, and such an impossible thing for him to do if he were Awake, that I knew it would work perfectly. I just had to figure out how to revive Shara afterwards. I toyed with the idea of faking her death; but in the end I decided, just as she and Hawthorne did, that the test wouldn't prove anything unless it demonstrably killed her (BTW, this is a good place to clarify that Shara didn't expect to be revived). But since she had to come back to life afterwards, that let out a gory death. Thus I settled on a lethal injection. Paul's test of the ingredients proved to him, and to my readers, that it really worked. But then (You see what a complicated thought process this was?) I had to figure out how Shara could be revived after the lethal injection. The magic ingredient in the saline is what the fine folks over on io9 call "handwavium."
So anyway, Shara's alive, Awake, and in a better position than anyone else to bring down Hawthorne. That's not to say she'll succeed, or whether anyone else will come along for the attempt. But at least you get to see the real Shara again after a long, dark stretch with her evil twin.