In His sovereignty, God chose to give both the angels and mankind free wills to make choices. Both angels and mankind fell when they chose their own will instead of God's will. God knew the risks of entrusting freedom to His creatures, but He also knew that relationships and love could only exist through the freedom of the will.
Love is a choice.
Unfortunately, freedom allowed sin to enter the picture and produce suffering and death. Sin also gives Satan an opportunity in peoples' lives. Sin allows Satan to rule for the time being upon the earth. But sin and Satan cannot stop God's will from being accomplished. In fact, the Lord can take anything and use it to accomplish His purposes.