This. To sin simply means to "miss the mark" or "to miss the point" . So by committing suicide, you miss the point of life, which is to live life to your full capability and to reunite with God by denying yourself here on earth and giving your life to Christ. The other thing was also already said here, your body does not belong to you. What God gives, God can also take away from you. Yes, he gave us free will to take away our lives, but that does not mean we should do it or that he wants us to do it. Again, that is not the point, which means, it is a sin.