early data mining of the next patch indicates sylv had something to do with xal, and that's what lead her to send horde forces to where azshara ends up being.
i don't think it's about respect, merely pragmatism - sylvanas is and always has been portrayed as being largely reactionary, almost everything she does is in response to something. even when she's scheming, it's because something else triggered her into doing so, she hasn't ever really thought up anything on her own.Nathanos is merely a devoted cultist, it's all sylvanas how come you think she wouldn't do that? Nothing currently indicates she has any sense of respect towards others, everyone is merely a puppet in her grand scheme.
so pre-emptively sending out assassins after thrall for "reasons" doesn't terribly fit her character, but it certainly fits that of nathanos.
not arguing it's the case, but it's a random fan theory that could possibly explain things.