In a very true sense, all sin is a mystery. And the difficulty is greater with the greatness of the guilt, with the smallness of the motive for doing wrong, and with the measure of the knowledge and graces vouchsafed to the offender. In every way the treachery of Judas would seem to be the most mysterious and unintelligible of sins. For how could one chosen as a disciple, and enjoying the grace of the Apostolate and the privilege of intimate friendship with the Divine Master, be tempted to such gross ingratitude for such a paltry price? And the difficulty is greater when it is remembered that the Master thus basely betrayed was not hard and stern, but a Lord of loving kindness and compassion. Looked at in any light the crime is so incredible, both in itself and in all its circumstances, that it is no wonder that many attempts have been made to give some more intelligible explanation of its origin and motives, and, from the wild dreams of ancient heretics to the bold speculations of modern critics, the problem presented by Judas and his treachery has been the subject of strange and startling theories. As a traitor naturally excites a peculiarly violent hatred, especially among those devoted to the cause or person betrayed, it was only natural that Christians should regard Judas with loathing, and, if it were possible, paint him blacker than he was by allowing him no good qualities at all. This would be an extreme view which, in some respects, lessens the difficulty. For if it be supposed that he never really believed, if he was a false disciple from the first, or, as the Apocryphal Arabic Gospel of the Infancy has it, was possessed by Satan even in his childhood, he would not have felt the holy influence of Christ or enjoyed the light and spiritual gifts of the Apostolate.
At the opposite extreme is the strange view held by the early Gnostic sect known as the Cainites described by St. Irenaeus (Against Heresies I.31), and more fully by Tertullian (Praesc. Haeretic., xlvii), and St. Epiphanius (Haeres., xxxviii). Certain of these heretics, whose opinion has been revived by some modern writers in a more plausible form, maintained that Judas was really enlightened, and acted as he did in order that mankind might be redeemed by the death of Christ. For this reason they regarded him as worthy of gratitude and veneration. In the modern version of this theory it is suggested that Judas, who in common with the other disciples looked for a temporal kingdom of the Messias, did not anticipate the death of Christ, but wished to precipitate a crisis and hasten the hour of triumph, thinking that the arrest would provoke a rising of the people who would set Him free and place Him on the throne.
In support of this they point to the fact that, when he found that Christ was condemned and given up to the Romans, he immediately repented of what he had done. But, as Strauss remarks, this repentance does not prove that the result had not been foreseen. For murderers, who have killed their victims with deliberate design, are often moved to remorse when the deed is actually done. A Catholic, in any case, cannot view these theories with favour since they are plainly repugnant to the text of Scripture and the interpretation of tradition. However difficult it may be to understand, we cannot question the guilt of Judas. On the other hand we cannot take the opposite view of those who would deny that he was once a real disciple. For, in the first place, this view seems hard to reconcile with the fact that he was chosen by Christ to be one of the Twelve. This choice, it may be safely said, implies some good qualities and the gift of no mean graces.
But, apart from this consideration, it may be urged that in exaggerating the original malice of Judas, or denying that there was even any good in him, we minimize or miss the lesson of this fall. The examples of the saints are lost on us if we think of them as being of another order without our human weaknesses. And in the same way it is a grave mistake to think of Judas as a demon without any elements of goodness and grace. In his fall is left a warning that even the great grace of the Apostolate and the familiar friendship of Jesus may be of no avail to one who is unfaithful. And, though nothing should be allowed to palliate the guilt of the great betrayal, it may become more intelligible if we think of it as the outcome of gradual failing in lesser things. So again the repentance may be taken to imply that the traitor deceived himself by a false hope that after all Christ might pass through the midst of His enemies as He had done before at the brow of the mountain. And though the circumstances of the death of the traitor give too much reason to fear the worst, the Sacred Text does not distinctly reject the possibility of real repentance.
from http://www.newadvent.org/cathen/08539a.htm
