https://en.wikipedia.org/wiki/Gospel_of_Judas
The Gospels make Judas out to be just a bad guy generally, even before he supposedly betrayed Jesus. They say he was a thief and that he didn't care about the poor, for example. Jesus even says outright that he's a devil. Then, you have this conflicting gospel of Judas. They can't both be true, can they?
Speculation time:
According to the book of Acts, things got ugly fast in the fledgling church. When Ananias and Sapphira sold their land and gave some of the proceeds to the Apostles, Peter demanded to know if they'd handed over
all the money from the sale. Rather than telling him to keep his nose out of their business, they lied to Peter, and they were killed for it. After that, "Great fear seized the whole church and all who heard about these events."
If there's any bit of truth to that story, I imagine many didn't much like the direction Peter and the Apostles were taking the church, such as employing violence and threats to keep the congregants in line. It makes sense that some people would break off, and come up with alternate interpretations of the basic gospel story that were wildly different from the Apostles' version.
= new reply since forum marked as read