Sylvannas is just pissed she didn't kill Arthas...
TBH I don't see why soo many are surprised by Sylvannas' actions in Cataclysm...Her stories been pretty transparent since Vanilla. She only joined Horde as a means to kill Arthas, she has been pretty obsessed and clear of this. Now he's dead what has she got? Nothing but power. The only reason she's resurrecting the fallen as Forsaken is to cling to the power. Her whole spin on 'Preserving the Forsaken' is clear BS. Majority of the Forsaken don't care for their races existence...They just wanted revenge and now bitter towards the living. TBH I have predicted a Forsaken civil-war since Vanilla...Now Arthas is dead why follow Sylvannas?