Why would God, if He knew Adam and Eve would sin, put a tree in the garden that would sentence them to death if they ate from it? 

We know God is sovereign, and there also is need for humanity to obey. A love without choice is not love at all, so God gave us the opportunity to obey out of love. And yet, Jesus was not God's plan B. Even as consequences were being discussed of Adam and Eve's sin, God tells them that there will come a time when a Messiah will come to redeem what had been done and make all things right again.