Yeah, that's true. However, Alak-Hul is the foster son of Oryx. Alak-Hul probably meant a great deal to him, making his betrayal sting all the more. In addition, the rebellion would've been devastating from a military perspective. Think about it; Alak-Hul's rebellion meant that a sizable chuck of Oryx's army has essentially disappeared, and is now resisting him. In addition, Oryx is constantly pushing the frontiers of his empire, meaning that he's already thinly stretched. Then, he has to contend with his sisters, Xivu Arath and Savuthun, who would've aggressively pushed at the first sign of weakness. In other words, the rebellion had a very real chance of ending billions of years worth of military conquest.

Alak hul was a very competent general of Oryx, so maybe Oryx was disappointed because he was losing someone so useful.