There is no such thing as "sin". None. Zip, zero, nada... It's been one of the most tragic and erroneous concepts ever made up in the mind of humankind. It's never existed and never will exist.
In the original language, it meant "to miss the mark, not perfect" and signified being out of harmony, out of right-relationship with all human beings and creation (thus with God too).
1) There is no such thing as perfection. There has never been anyone or anything on this planet that was perfect. Humankind doesn't even know what it is, as is beautifully exemplified in the feeble attempts to describe the "perfection" of Heaven in the Book of Revelation... (For example, the book of Revelation describes a place where there is no pain and the sun is always shining, there is no night. Pain is necessary for survival and even growth, have you seen what happens when people do not have pain? And anyone who has lived in the Arctic knows, the 24/7 sun is HELL. No one can sleep, after 3 months, people are grouchy from sleep deprivation and fist fights break out - the whole Arctic heaves a huge sigh of relief when the sun finally goes down! The body needs the darkness to process melantonin and to heal. What's true physically, is true spiritually - we need pain and we need the darkness, just as much as we need wholeness and sunshine. We need the polarity.)
2) Where in the world would the change, growth and the development of a human being coming from if it were not for our "missing the mark", if it were not from our mistakes? How many times did you have to fail tying your shoes before you finally mastered the task? How many times did you have to wobble and fall before you learned how to balance and ride a bike? The same is true with learning the art of friendship and romance - we have to learn how to set boundaries, how to negotiate, how to argue fairly, etc... All learning experiences. Our successes are built on the foundations of our failures and our mastery is built on our imperfections. Funny how we have been made this way.
3) Evil exists. It most certainly does, but it is not explained by the ridiculous concept of "sin". It is much more than "imperfection", "missing the mark"... Evil has much more to do with the state of prejudice and hatred in the heart, with the absence of empathy and the intention to harm. No wonder Jesus said, "Blessed are the peacemakers."