The so-called "problem" of evil is a stumbling block in the faith of many people. To a lot of people, it's enough to stop them even believing in God, but even for us as Christians, it's often unfathomable that God should allow evil to exist.
The Bible tells us three very important facts that tell us why God allows evil to exist (for now). First, God hates evil. Second, we are evil. Third, God loves us. God, therefore, faces a dilemma. If He destroys all evil today, He could be destroying someone who will repent tomorrow. Thus, as Peter says, what we as humans see as being slow to destroy evil is actually God being patient enough to give just one more sinner time to repent.
Evil can never dwell with God. One day, God's patience will run out, and evil will be destroyed. Tragically, that will result in the destruction of many human souls. But if just one more soul can be saved, is it not worth holding that off?