I can answer your question in another way. If God were to put a stop to evil actions, what do you think that would entail? By God's standards, we're all guilty many many times over for various sins, so the only thing God could do would be to judge and punish every single person accordingly right now. And it's kind of arbitrary, because you could say, why didn't you just judge everybody 100 years ago? or why didn't you just judge Hitler, and Stalin and so on. From a Christian perspective, the fact you and I are alive right now, is because God has the grace not to pull the plug on creation. The acts of violence you see in the world are the tip of the iceberg of human wickedness. Imagine what a murderer's heart looks like to God. In fact, God says we're all murderers, because we've all hated somebody with the fantasy or intention of harm towards them. In God's eyes, who is pure, that is murder of the heart.
God's standard of what constitutes evil is just much much purer than our own, but his mercy is also much greater than ours; it's greater than we can imagine. The fact God allows for these violent acts is the same reason he allows you and me to exist. Mercy. Suffering is an extension of the same grace that saw Christ suffer and die on the cross. In fact, suffering is God's wisdom. Suffering says that God loves the world so much he will endure it even though it grieves him so bitterly. But the Bible does clearly state that there is a day coming when God will judge man, and that day belongs to the Lord.