That is a question that bothers all who live on this earth, whether they believe in God or not. I am a Christian and I can only answer this question from a Christian perspective. I would speculate that if you believe in some other god that it does not care, is not around, or does not have the power. Of course if you are an atheist it is a mute question.
The Bible says that God is love. It is no stretch to see that love needs someone to love to be complete. Nobody wants to love someone that does not love them and nobody wants to love a machine. This is as true of God as it is of us.
If God made man and made man love Him it would be the same as making a machine. God had to give man the power to reject Him, or not love Him. This is the first of the two royal laws, (Deu 6:5 NIV) “Love the LORD your God with all your heart and with all your soul and with all your strength.” It is not hard to do when you realize everything God has done for us.
Here is where the problem lies. If God gave mankind the power to sin and the ability to chose to sin or not to sin and then stops him, or her from sinning, then He really did not give either the power, or the choice. Can you see that God tied His own hands and can not, or rather will not make someone do what is right? He even tells His followers not to interfere in the choice (Rev 22:11 NIV) Let him who does wrong continue to do wrong; let him who is vile continue to be vile; let him who does right continue to do right; and let him who is holy continue to be holy.”.
If you set up a scientific experiment and then changed it in any way, while it was being conducted, it would invalidate the results. If you view this life as an experiment you can see that God can not interfere.
We know that God knows everything, so God, knowing the future, already knows if we will reject Him or love Him. So I would assume that we are here to prove to ourselves whether we accept, or reject God.
If you reject God then you will probably reject His second royal law (James 2:8 NIV) “If you really keep the royal law found in Scripture, “Love your neighbor as yourself,” you are doing right.” But if you keep the second law and do not keep the first will you live? Do you love God?
Anyway, wars and all crimes come from not obeying the second law, and if God would stop even one person from sinning then to be fair He would have to, or had to, stop everyone. (Whoops, check kclick’s comment)