Now leave me alone: This sounded like a command, but it was actually an invitation. God was giving Moses an opportunity to pray for the people. Moses was to stand before God as an intercessor (a person who speaks to God on behalf of others).
The people deserved to be destroyed because of their sin. However, God was willing to spare them if Moses continued to pray for them.
I will make you into a great nation: God told Moses that he could begin again with him alone. God could abandon the other descendants of Abraham and build a new nation from the children of Moses.
This was likely a test. It showed whether Moses understood God’s character and his promises.