Sin and free will explored
Christians believe that God gave us free will, that is we can make our own decisions and go our own way. God's world is basically good, but we have the option to make good or bad decisions about our lives.
The story of Adam and Eve in Genesis gives us one explanation of how sin might have entered the world at what we call 'The Fall'. Look up Genesis chapters 2 and 3. The idea of 'original sin' is believed by many people - that is we are all born with the tendency to sin, and that we cause suffering ourselves.
However, as we seem to start life with a tendency to sin, we have free will and we can choose to follow God's way of love for us and for our world.