I'm not a bad person. Why do I need a mediator to atone for my sins?

No one said that you were a bad person. However, everyone has committed at least one sin against G-d (see Psalms 14:3; Ecclesiastes 7:20). Abraham (see Genesis 12:10-20), Moses (see Numbers 20:7-12), and even King David (see 2 Samuel 11:1 through 12:14) sinned against G-d. Isaiah went so far as to say that even our righteous deeds are like shmattes (filthy rags) to G-d (Isaiah 64:6). Our rabbis call this sin nature of man the yetzer hara (evil inclination). Every one who has ever committed a sin needs to have atonement made for that sin. In other words, since sin is rebellion against G-d—even if it's just a little white lie, we need a way to set things right with G-d. Otherwise, the consequences of sin are separation from G-d (see Isaiah 59:2; Jeremiah 31:30; cf. Daniel 12:2).

Yeshua serves as mediator between man and G-d—much like Moses, the Prophets, and the Priests did (see Exodus 20:18-19; Numbers 17:9-13 [16:44-48 in some translations]). Thus, by believing in Yeshua the Messiah's atonement for sin, G-d forgives our sins and we are at one with G-d once again.

Additional information can be found in the book "The Death of Messiah" and in the booklet "Are There Two Ways of Atonement?".