The surname is unexplained. The commonest derivation reads it as ish Kerioth, a man of Kerioth, a Judaean town, which would make him the only non-Galilean of the twelve. A rival derivation connects it to sicarius, the Latin for dagger-man, used of a faction of first-century Judaean assassins, but that faction is attested only later than the events, which weakens it. Neither is secure.
In 2006 the Gospel of Judas, a second-century Coptic text within the Sethian Gnostic orbit, was published to considerable publicity. The National Geographic team presented it as portraying Judas as the favoured disciple who betrayed Jesus at his own request, freeing him from the body. That reading was challenged almost immediately and in detail by April DeConick, who argued the translation had inverted key terms: the text calls Judas a daimon, and says he will be cursed and will not ascend, so that far from rehabilitating him it makes him the worst of the disciples in a text that despises all twelve. Her reading has been broadly persuasive. In neither reading is the text a historical source about the man; it is second-century polemic.
The most consequential later use of Judas is not textual at all. From the medieval period his name was made to stand for Jews generally in European art, drama and sermon, and the passion plays that did this were an engine of antisemitic violence. The Gospels present him as one of the twelve, all of whom were Jews, and none of them draws that inference. It was supplied by later readers, and it killed people.