Might as well jump in too.
1. As a starting point, consider Pascal's Wager.
en.wikipedia.org
---------A reason to initially consider believing in the existence of God.
2. While you're at it, consider what theologians term "The Problem Of Evil".
en.wikipedia.org
---------Why an all-powerful God AND evil exist.
3. Finally, consider some explanations as to why evil is permitted to exist.
Plantinga's Free Will defense
en.wikipedia.org
---------God restrains from interference in world affairs so as to not violate people's free will.
Boyd's Spiritual Warfare explanation
In light of the profound evil being experienced by the people of Paris and countless other locations around the world, we thought we would raise again the question that many ask when things like this occur: Why? Of course, Greg has spent much of his writing and speaking energy addressing this...
reknew.org
----------Satan is at work in the world initiating evil; God is battling him.
The above is only an initial introduction to why incredible evil is often not restrained.
Consider studying the "Problem of Evil" for answers.
I do acknowledge being a non-Religious Right Evangelical. We DO exist (Tony Campolo, Ron Sider, etc).