Maybe I've not paid close enough attention to the dialogue in Nighthold, but why do we fight Krosus?
It doesn't seem that his presence would impede our assault on the Nighthold, as he's a bit out of the way. Him blocking our only route to the Tomb of Sargeras might make sense (a la Kologarn in Ulduar), but we're not going there yet. Instead, it's detour, kill boss, backtrack.
Is it merely revenge for Tirion?