Hail, Rabbi. I have no doubt that Judas, as if trembling for his Master's danger, pretended by these words to have some feeling of compassion; and, accordingly, in Mark a pathetic repetition is expressed, [1] Rabbi, Rabbi. For though he was impressed with the majesty of Christ, still the devil so fascinated his mind, that he felt assured that his treachery was concealed by a kiss, and by soothing words. This salutation, or exclamation, therefore, was a pretense of compassion. I offer the same opinion about the kiss; for though it was a very common practice among the Jews to welcome friends with a kiss, yet as Judas had left Christ but a little before, he seems now -- as if he had become suddenly alarmed at his danger -- to give the last kiss to his Master. Thus he excels the rest in the appearance of affection, when he appears to be deeply grieved at being separated from his Master; but how little he gained by his deception is evident from Christ's reply.

Footnotes

1 - "Il y a une repetition comme d'un homme parlant de grande affection;" -- "there is a repetition, as if by a man who spoke from strong feeling."

Hail, Master - The word translated "hail," here, means to "rejoice," to have joy, and also to have "cause" of joy.
It thus expresses the "joy" which one friend has when he meets another, especially after an absence. It was used by the Jews and Greeks as a mode of salutation among friends. It would here seem to express the "joy" of Judas at finding his Master and again being "with him."
Master - In the original, "Rabbi." See the notes at Matthew 23:7.
Kissed him - Gave him the common salutation of friends when meeting after absence. This mode of salutation was more common among Eastern nations than with us.

And forthwith he came to Jesus,.... As soon as ever he appeared, before the rest could come up; for, he went before them, as Luke says, and that not only as, their guide to direct them to the person they wanted, but he separated himself from them, that it might not be thought that he came with them, or belonged to them:
and said, hail, master; and kissed him. Just as Joab asked Amasa of his health, and took him by the beard to kiss him, and smote him under the fifth rib, 2-Samuel 20:9. The salutation he gave him was wishing him all health, prosperity, and happiness. The Syriac version renders it, "peace, Rabbi"; and the Persic, "peace be upon thee, Rabbi"; which was the very form of salutation the disciples of the wise men gave to their Rabbins,
"Says (n) Aba bar Hona, in the name of R. Jochanan, in what form is the salutation of a disciple to his master?
, "peace be upon thee, Rabbi".
In Mark 14:45, the word "Rabbi" is repeated, this being usual in the salutation of the Jewish doctors; and the rather used by Judas under a pretence of doing him the greater honour, and of showing the highest respect, and strongest affection for him. So this deceitful wretch still addresses him as his master, though he was now serving his most implacable enemies; and wishes him all peace and joy, when he was going to deliver him into the hands of those that sought his life; and to cover all, kissed him, as a token of his friendship and the sincerity of it. It is rightly observed by Dr. Lightfoot, that it was usual for masters to kiss their disciples, particularly their heads; but then not for disciples to kiss their masters: of the former there are many instances in the Jewish writings, but not of the latter: yet, I can hardly think that this was done out of open contempt and derision; but under a pretence of respect and love; and even as being concerned for his present case, and as condoling him under the circumstances he was now likely to be in, through an armed hand, which was just upon him; and which he, by this artifice, would have suggested he had no concern with,
(n) T. Hieros. Shebuot. fol. 34. 1.