Although I was the first (I think) to say that perhaps Seht regrets the decision, I'm equally devoted to the possibility that he is has an air of impatience about him, or maybe he is internally scolding Dagoth for being so rash in using the Tools without the Wraithguard. It really could be anything. I do wish we had more of him, though. But, then again, he is the Mystery of Morrowind.
While we are on this
mystery, another explanation is that Sotha Sil is impatient with the other two's love of ceremony. There the advlterous two are, basking in the symbolism of murdering their King, while 'Sil is left holding Wraithguard waiting for them to finish. If ever there was an anolytic mind, it belonged to the Mystery of Morrowind. "When you're finished.."
Alternatively, and it is the answer I like the best, Seht's unwillingless to watch the red act take place is the one element in the picture to restore ones faith in merkind.