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<<I know Slade can be a real asshole, but does this strike anyone else as out-of-character (especially given how he looks horrified in #12 when he sees what Rose did)?>>
Nope, that's the way Geoff Johns intended it, as far back as TEEN TITANS #12 where Bart Allen and Cyborg surmise that Deathstroke had been pumping Rose with the Deathstroke serum, and it made her psychotic. Also confirmed in person by Geoff himself. "It's the serum," he said.
read her history: http://titanstower.com/source/whoswho/rose.html
The new flashback merely relates "what happened next", as the serum started to wear off. When Rose was under that serum's influence, her life was a living nightmare. That's why she's as damaged as she is now. Sean McKeever has done an excellent job of continuing Rose's story where Geoff left off.
And yeah, I'm not a fan of the "new" totally evil Deathtroke. I liked the morally ambiguous Slade Marv Wolfman created. Not asshole that preys on young girls. But that's what we get now.