We were all, both angels and humans created with a free will. God did this because he did not want us to be like robots, premanualed to love and obey him, because love simply can not be forced. So without free will, no love.

Now, God probably knew when he created all things that sin (disobediance) would enter this world. First by one of the highest created angels, Lucifer, which because of his pride desired to be God and was together with a mass amount of angels cast out of heaven and down to earth. This was how evil entered the world, not because God wanted or made it so, but as an inevitable consequence of creating angels and us with a free will.

God could have chosen to wipe out Lucifer and the angels that took his side, but this would only have led the rest of the angels to continue obeying God out of fear and not love. The same goes for us humans when the first created humans were put to the test to see whether they would choose to trust God in all things or not.

Man chose to follow the Devils temptations in stead of trusting in God. So because we chose not to trust God, the creator of all things, sin entered the world. Sin means not to obey and trust that God, the almighty, knows what is best for us. We chose to believe we knew better than Him!

You could also propose the question as to:

Why did God decide to create angels and humans when he knew what would happen?

Now think of a loving parent who decided to procreate even though they knew that child were going to rebel sooner or later. Love takes risks. God chose not to give up on us, he found a way by the sacrifice of his only begotten son, Christ, in order to give us a chance to get back to the garden again and not having to die. We were created with the purpose to live side by side with God for eternity and not to just die and decompose. Ezekiel 33:11Say unto them, As I live, saith the Lord Jehovah, I have no pleasure in the death of the wicked; but that the wicked turn from his way and live: turn ye, turn ye from your evil ways; for why will ye die.

God wants us to understand with our entire soul that everything he does is for our own and the universe benefit and to love, trust and obey him for those very reasons.