If we say that we have no sin, we deceive ourselves, and the truth is not in us. Fellowship with God requires that we acknowledge the truth concerning ourselves. To deny our sinful nature is self deception. You aren't fooling God. Notice that John makes a distinction between sin and sins in verses 8 and 9.
Sin refers to our corrupt nature.
Sins refers to evil things we have done. Actually what we are is a lot worse than what we have actually done but thankfully, Christ died for what we are as well as what we have done and will do as long as we live in this body. One of the worst things we can do is to judge other's sins. We tend to overlook our own at their expense. I know of some people who are terribly taken up with the sin of abortion to the extent that they hate those who perform them.