Sinning is not about creating substantial, unjustified harm to your fellow man. It is about offending God. Not believing that you are a sinner is in itself the sin of pride. You might want to read the Ten Commandments again and then read Jesus' words about them. As an example, Jesus said that a man who looks at a woman lustfully has committed adultery in his heart. There are many other examples. Have you ever gossipped, had a rude thought about someone, wished for something someone else has, or argued with someone because you wanted your own way? These are sins (backbiting, detraction, envy, lack of humility), although not as serious as murder or stealing. The seven deadly sins are pride, anger, envy, covetousness, gluttony, lust, and sloth. There are a lot of sub-categories to each of them. Most of the sub-categories have to do with how we think and not so much our actions. Offenses against love for our fellow man begin in our minds and hearts, and even if we never actually "do" anything harmful to someone, just thinking harmful things is still a sin. Jesus said, "What comes out of a man is what defiles a man. For from within, out of the heart of man, come evil thoughts, fornication, theft, murder, adultery, coveting, wickedness, deceit, licentiousness, envy, slander, pride, foolishness. All these evil things come from within, and they defile a man." Mark 7:20-23
I'd take it a bit further. Have you ever been God? If no, then you are a sinner. Sin isn't something we do - it's something we ARE.
I wonder how many God-fearing souls here have sinned and thus plucked out their eyes and cut off their hands? Or do we just obey the word of God as it is convenient to do so.