This is the *least* senseless possible explanation. First, you have to accept their explanation as to *how* Bhaal revived despite the outcome of ToB. That is also an unsquarable circle, but let's accept it for sake of argument. If Sarevok were restored with his old viewpoints, he would not willingly serve Bhaal, as this was an option he forewent in favor of other paths previously. The LEAST senseless explanation (which still has holes, just the minimum number of holes) is that he is like the spirits behind him and simply a force in motion that lacks any true agency as the name "echo" implies. He showed an awful lot of exposition for his viewpoint for having a lack of agency. The *least* senseless explanation to this is that it is not so much the result of a thought process, as it is just a recorded script, as one would find in a voicemail or answering machine, that just gives a scripted output for a scripted input and these were imparted by Bhaal (this is just the in universe equivalent of "he was programmed" the same way he was literally programmed in the game code). This just upgrades him to an extremely sophisticated golem with pre-programmed protocols, which is the *least* senseless answer.