"Bear in mind that our Lord's patience means salvation, just as our dear brother Paul also wrote you with the wisdom that God gave him. He writes the same way in all his letters, speaking in them of these matters. His letters contain some things that are hard to understand, which ignorant and unstable people distort, as they do the other Scriptures, to their own destruction." NIV translation
Why does God allow the evil to continue and even to increase? The simple answer is found here as Peter says that it is patience and the opportunity for people to be saved. Peter and Paul had some early disagreement but they were able to settle their differences and work together to further the kingdom as we see Peter defending him here. In light of the fact that Jesus is coming soon, we should follow this example and just get about the business of sharing Jesus with as many people as we can.
"Therefore, dear friends, since you already know this, be on your guard so that you may not be carried away by the error of lawless men and fall from your secure position." NIV translation
Since we know the truth, Peter urges us to hold on to it even in the face of the false prophets. Many have tried to use this verse to say that we can lose our salvation but, when he talks about "fall from your secure position", he is not talking about losing salvation. In Christ, our position as children of God is secure and God does not toss His children out on the street.
Peter is talking about those that have followed these false prophets into a religion instead of a relationship with Jesus Christ. Our position is sealed with the Holy Spirit and that Spirit will not allow you to be "carried away by the error" as He is the Counselor that Jesus promised.
"But grow in the grace and knowledge of our Lord and Savior Jesus Christ. To him be glory both now and forever! Amen." NIV translation
Peter ends this second letter by urging us to grow in our knowledge of Jesus and His grace. The more we learn about our faith the harder it is for us to be deceived by the schemes of the devil.