Maybe I'm letting my hatred of the EN raid seep through and it's been a while since I read Well of Eternity.
He was definitely more impressive in the book and he did some terrible things in both the book and the game, however he doesn't impress me as much as many other characters.
Someone made a theory that it is actually Gul'dan dying, since he was from an alternate dimension, the Arbiter cannot judge a soul twice. The timing does line up.
Do I like it? Hell no. Sick of Gul'dan ruining things that are pretty! > However, we did see Argus' soul was blue, not red, after he is defeated, so...