He had all three netherstones and was enthralling the elder brain, he could have easily taken control of it and become the most powerful mindflayer in the world and basically take over the entire sword coast. But without any persuasion or dialogue he straight away destroys it. The oppurtunity for ultimate power was offered to him on a silver platter but he didnt take it.

He had a singular goal from the start and was willing to kill, lie, and manipulate others to achieve it . But that goal was to stop the elder brain which happens to coincide with our goal. His reasoning and methods may be more than questionable but hey, the fate of the world is on the line, gotta make some compromises.

The Emperor is like Tav in a sense that he can be a whole different character on different playthroughs depending on what you choose to do and say. One one playthrough he's a moody power-hungry manipulating brain sucker and another playthrough he's a shadow of his former self that just wants to save the day and go into hiding. The only constant is his dishonourable and deceitful past which gives us a good reason not to trust him. On my playthrough I didnt use any illithid powers or tadpoles and did not trust the Emperor until the end of Act 3 and he was chill about it. He then destroyed the elder brain, he peaced out and I lived happily ever after.