Albert Barnes on Matthew 26:50

And Jesus said unto him, Friend. It seems strange to us that Jesus should give the endeared name friend to a man that he knew was his enemy, and that was about to betray him. It should be remarked, however, that this is the fault of our language, not of the original. In the Greek there are two words which our translators have rendered friend: one implying affection and regard, the other not. One is properly rendered friend, the other expresses more nearly what we mean by companion. It is this latter word which is given to the disaffected labourer in the vineyard: "Friend, I do thee no wrong," (Mt 20:13) to the guest which had not on the wedding garment, in the parable of the marriage feast, (Mt 22:12) and to Judas in this place.

Wherefore art thou come? This was said, not because he was ignorant why he had come, but probably to fill the mind of Judas with the consciousness of his crime, and by a striking question to compel him to think of what he was doing.

(u) "Friend" Ps 41:9, 55:13