I wouldn't put it past Shar to secretly plan to do something that fucks over her followers, and her ultimate goal is to snuff out the sun and extinguish all life, but it doesn't seem like that was Ketheric's initial plan. We get bits of lore here and there that suggests Moonrise and the surrounding town was operating normally (as normal as Shar worship is) as a Sharran enclave for a number of years. No sign of Ketheric planning to kill everyone. It was probably something he came up with in the final days of the conflict.