God, who is planning another mass murder, is worried that Abraham might try to stop him. So he asks himself if he should hide his intentions from Abraham. (18:17)
Respond: This is because if we turn to (Gen 17) where Yahweh God promised that Abraham will become father of multitude, and now in (Gen 18) God wants to destroy Sodom and Gomorah this will make Abraham think about the covenant, that Yahweh God promised and how he can break it. This is why God Almighty in (Gen 18:18-20) clearly says that Abraham will become a great nation.