Calia provoked Sylvanas by asking the UC people to defect over to the alliance and if i remember correctly even asking them to run over. My memory is not very good at this point because its been almost a year since I read the book.
Nonetheless, yes Sylvanas was still in the wrong, i did not deny it as i mentioned, to killing those undead either way. However, I still think that what killawar said earlier made some sense as well. That she is in a perilous position and ascended to the throne of warchief at a rather bad time.
All in all, my take in this is that Sylvanas has ulterior motives, but how she achieves them is questionable. Pyromancer had one interesting theory on why she is doing this but whether or not that holds true, we’ll find out soon enough I suppose!