Sin is considered a spiritual evil. Evil is the corruption of good. God desires us to be good that is why he created us because he loved us, he saved us because he loved us, and continuously gives us sanctifying grace because he loves us. We all are inclined to do good but it is for us to decide if it is for our own selfish reason or for the common good. By doing this we would attain our "end" or purpose.
Hope this helps
