Genesis questions...

I dont understand why God would be ok with Abraham sending away Hagar and Ishmael.
It just doesnt make sense to me right now that God would agree with Sarah to have them sent away. But wait, not just did they get sent away, but Ishmael was going to be killed. then God had mercy on Hagar and the child's weeping. SO God had them sent away, to open the opportunity to show mercy. Im not sure what this means or why He would do this...