Headcannon theory: Sarevok was successfully manipulated to his death by Melissan during BG1.
Spoilers obviously:
>!Throughout BG1, Sarevok believes that he has discovered a loophole in Alaundo's prophecies, which he believes will allow any of the Bhaalspawn to become Bhaal Mk2 if they cause enough murder. His plan is to manipulate Amn and Baldur's Gate into a war, causing massive amounts of death and destruction, while he hides out in the Undercity, performing a ritual and hoping to spontaneously erupt into Bhaal reborn. !<
>!The problem is that this is complete nonsense, and has no prospect of success. ToB makes it clear that the Bhaalspawn are not Bhaal's heirs, they're a kind of temporary storage unit, preserving enough of his divine essence after his mortal death, restoring the essence to the Abyss on their deaths, and allowing Bhaal to be reborn when they all die. They're all supposed to die in a mass sacrifice at the Temple of Bhaal, but the Harper assault disrupts this plan, so this is now a long-term, rather than immediate plan. !<
>!From ToB again, Melissan has been granted temporary use of this essence, with the purpose of restoring Bhaal once she has control of all of the essence. She plans to betray Bhaal and steal the essence, but that isn't immediately relevant. What is important is that Melissan and/or Bhaal need as many Bhaalspawn to die as quickly as possible. This is the whole plot of ToB.!<
>!Sarevok's plan in BG1 is not just implausible, it's functionally suicidal. He has no real plan to avoid detection after causing the war, nor any plans to escape. As far as he is concerned, he doesn't need them. Exactly how he ends up dead is besides the point, but if he wasn't killed by Gorion's Ward, he would have been killed sooner or later by the Flaming Fist, the Iron Throne, the Shadow Thieves, or the Amnian army. !<
>!What he absolutely is convinced of is that he needs to kill as many other Bhaalspawn as possible, unless one of them steal his crown. So- Sarevok ends up dead, can't become Bhaal, and might with luck take a few of his siblings with him. !<
>!You may start to see where this is going- none of this actually serves anyone's interests except Melissan and/or Bhaal. !<
>!So how is this happening? Eyes on https://baldursgate.fandom.com/wiki/Winski_Perorate , who tutors Sarevok in Bhaal-lore and plainly is capable of strongly influencing his decisions. I'm suggesting that he's not the dupe he appears, but is really acting for Melissan. As he says "how could there not be a place in history for the architect that shaped the actions of the ascending Lord of Murder? " He does not say in so many words that he means Sarevok. !<
>!This is all very much in Melissan's MO. She poses as a tutor and a guide to Bhaalspawn, leading them down the path to destruction while profiting from it in the long run. It's completely plausible that she has agents furthering those goals alongside the more simplistic methods the Five use. !<
>!Come to think of that- did she point Irenicus in the direction of Gorion's Ward when they improbably survived? !<
TL;DR >!Melissan has sent Winski Perorate to pose as Sarevok's mentor, with the ultimate agenda of getting himself killed and, if possible, taking some other Bhaalspawn along the way, something deeply consistent with the canonical story of both BG1 and ToB.!<