Are there any Christian sects that believe the Old Testament was written (inspired) by Satan or that these scriptures are evil in some way? I wonder that if there are Christians that interpret the Old Testament as something so false/deceitful that it may be the work of the Devil himself in an attempt to lead humanity out of God's true path.
Catharism (a Gnostic Christian movement in the 12th to 14th centuries), held the belief that the Old Testament God was Satan (and thus that the scriptures were inspired by Satan). From Wikipedia:


  The idea of two Gods or principles, one being good and the other evil, was central to Cathar beliefs. The good God was the God of the New Testament and the creator of the spiritual realm, contrasted with the evil Old Testament God—the creator of the physical world whom many Cathars, and particularly their persecutors, identified as Satan.


The Cathars were branded heretics by the Catholic Church. After sending missionaries failed and the Pope's legate was martyred, the Albigensian Crusade was launched, killing most of the Cathars and forcing the survivors to recant and rejoin the church.

Another question on this site asks: What was the explanation for why Catharism identified the Old Testament God as Satan?