Gods thrive on two things: (1) Doing the thing that the particular god is about, and (2) Belief in that god.
Sarevok wanted to become a murderer on a mass scale and set off a war the like of which had not happened for generations, over no good cause since there was no good reason for a dispute between the nations (neither could caim self-defence: and even "natural competition over scarce resources with growing populations", often a reason which is considered reasonable, did not apply since the scarcity in question - the Nashkel Iron Crisis - was artificially manipulated.
Also he believed that killing off as many other Bhaalspawn as he could, would feed his own growing power, as the most powerful Bhaalspawn and thus the most powerful vessel of the essence of Bhaal in the area.
Also: As we later learn, many of the other Bhaalspawn were scattered over the area south of Amn, where Tethyr was by-and-large in a fairly chaotic state. Amn itself, in fact, had few - or, if there were many Bhaalspawn in Amn and in particular in Athkatla, they were already tracked down and eliminated.
Many of the southern Bhaalspawn became murderers, and sometimes warlords, in their own right. It is hinted in "Throne of Bhaal" that the uprising of the "Sythsillian Empire" which threatens Amn from the south (and of which increasingly growing reports are heard from the Town Criers during "Baldurs Gate 2: Shadows of Amn", starting from it being a minor uprising and finishing with it being a major threat), was the work of an unnamed half-ogre Bhaalspawn - who later fell to Yaga-Shura (one of the chapter bosses in ToB), but not before Athkatla was severely damaged.
Other Bhaalspawns were, in fact, innocent. There was still purpose in their existence: a murder requires a victim as well as a perpetrator. They were seen, of course, as targets by the more malignant-minded Bhaalspawn.
And not only by them: by the normal public, who came to fear and hate Bhaalspawns. Many innocent people who just happened to be Bhaalspawns, were murdered - and not even by other Bhaalspawn - for no reason other than their heritage, even if they had *done* nothing evil.
And thus, along with the whipping up of mass hysteria that led to wars with no good cause, we see another factor in the Bhaalspawn Crisis: ordinary people coming to believe in the idea that murder was a solution to their problems, whether murdering individual Bhaalspawn or making wars of aggression against foreign nations and thus taking part in mass murder (and, incidentally, having Bhaal, the god of murder, trespass on the territory of Tempus, the neutral god of battle.) The more that normal people believe in murder as a solution, the more powerful is the essence of Bhaal for someone to seize.
Sarevok himself realised that: had he succeeded, his army would have been the first to mobilize, and he might have taken out Amn, smashed through, and taken the fight to the Bhaalspawns of Southern Tethyr before they had fully come into their own power. And who knows what the result of such a fight might have been: Sarevok himself had learned the rituals to ascend to the vacant position of godhood, and might have achieved it before the true villain of "Throne of Bhaal" could get there.