As @Lord_Mathos mentioned, Nathanos looks to have hesitated when Sylvanas tells him to burn the tree.
This is the 2nd example in a short period of time where he does this. The first is when he challenges Sylvanas when she's killing her own citizens in Before the Storm.
Hmm
Nathanos got his own model. (Check) Nathanos got more screen time. (Check) Nathanos is a jerk but cares for his Dark Lady & Forsaken. (Check) Nathanos to be the new Leader of the Forsaken and carry on Sylvanas' orignal wishes is my guess for the coming future.
