The slavery arguement is only an arguement against a morally perfect God. A God who was the ultimate good being would not allow his chosen people to practice slavery. God in the bible gets very angry about people worshipping statues or being given the wrong sort of sacrifice. However, he doesn’t smite anyone for slavery, which is argueably a much greater evil. So, the God of the bible is not a morally perfect being.
That doesn’t mean he doesn’t exist, of course. The slavery arguement and the rape arguement* are arguements against God’s goodness, not his existence.
*rape is never explicitly condemned in the Bible and in some stories the Israelites are given permission by God to rape their enemies virgin women.