It it reasonable to kill innocents just because your own innocents were killed? Tyrande is a threat to all of the Horde not just Sylvanas. In Shadows Rising she called for the obliteration of the whole Horde. Vengence crosses the line when it threatens to wipe out entire peoples. Sylvanas burning Teldrassil wasn’t a one off but a series of vengences, the Night Elves almost wiped out the High Elves too long ago over vengence. Is it justified on either side? No.