Why is Derek the straw that breaks the horde's back?

[Treng-wyrmrest-accord (Treng)]

@Gharion-ragnaros

“Whole city of innocent civilians Burned down.”
I sleep.
“Your own troops blighted, raised and (possibly) dammed for all eternity.”
I sleep.
“One human is raised and mind controlled.”
Real sh@@.

The first two should be the straw that breaks the Horde’s back. The third should be the straw that breaks the Forsaken’s.