My heart goes out to you, for I doubt if anything is more painful and heartrending to a parent than losing a child. Over the years, I have seen it happen to some of my closest friends, and I know the heartache they have suffered.
But the most important thing I can say to you is that God's heart goes out to you. God knows what you are going through, and He understands the emotions that have overwhelmed you. After all, God knows what it is like to lose a child, for His only Son was nailed to a cross. The Bible says, "The Lord is compassionate and gracious, slow to anger, abounding in love" (Psalm 103:8). Even if you don't believe in Him, that does not alter His love for you.
No, I don't know why things like this happen, nor will we ever fully understand them until we get to heaven. But I do know this: God didn't cause your son's leukemia, nor did He take delight in His death. We live in a world that has been touched by evil and sin, and life is often hurtful. Jesus wept as He stood by the grave of His friend Lazarus (see John 11:35).
I also know this: It is far better to turn to God when tragedy strikes than to turn away from Him. God loves you, and He wants to comfort and help you. His promise is true: "I will give them comfort and joy instead of sorrow" (Jeremiah 31:13). Don't turn away from Him, but by faith open your heart to Christ.