We feel outrage, perhaps even anger, and we feel deeply incensed that any just universe could allow such an unjust and evil thing to happen.
If we have grown up believing in God or in some great Order operating in the universe, we are liable at that point to feel great anger at God, and deeply outraged that he (or she) is not the just and righteous being we originally believed.
None of them is entirely satisfactory alone, but together they do cast doubt on whether the existence of evil disproves the claim that God exists.
The first response to the problem of evil is the free-will defence.
But at some point it may happen in a life that someone you love deeply and someone you know to be a good person (perhaps this person might even be you) has the tragic misfortune of having to undergo a great and undeserved suffering.
When you see that happen, when you watch that good person undergoing such enormous undeserved evil, it violates every sense of justice we have understood up to that point.
It is almost as if they are in some sense trying to act as if that other person does not even exist.
In a similar way, it can also happen that when someone gets very angry at God for such a perceived violation of all the principles of justice, fairness, and good order in the universe, they can also furiously turn their back on God.
The classical "problem of evil" is one that has been around for millennia.
In one way it can be seen as purely a philosophical or academic quandary that could somehow be solved with an appropriate philosophical insight and argument.