Here are some insights into verses 17- 21 from St. Thomas Aquinas’ Catena Aurea, which compiles the commentary of various Church Fathers:
21. The Son of man indeed goes, as it is written of him: but woe to that man by whom the Son of man is betrayed! good were it for that man if he had never been born.
BEDE; The Lord who had foretold His Passion, prophesied also of the traitor, in order to give him room for repentance, that understanding that his thoughts were known, he might repent. Wherefore it is said, And in the evening he comes with the twelve.
And as they sat and did eat, Jesus said, Verily I say to you, One of you which eats with me shall betray me.
CHRYS. Where it is evident that He did not proclaim him openly to all, lest He should make him the more shameless; at the same time He did not altogether keep it silent, lest thinking that he was not discovered, he should boldly hasten to betray Him.
THEOPHYL. But how could they eat reclining, when the law ordered that standing and upright they should eat the Passover? It is probable that they had first fulfilled the legal Passover, and had reclined, when He began to give them His own Passover.
PSEUDO-JEROME; The evening of the day points out the evening of the world; for the last, who are the first to receive the penny of eternal life, come about the eleventh hour. All the disciples then are touched by the Lord; so that there is amongst them the harmony of the harp, all the well attuned strings answer with accordant tone; for it goes on: And they began to be sorrowful, and to say to him one by one, Is it I? One of them however, unstrung, and steeped in the love of money, said, Is it I, Lord? as Matthew testifies.
THEOPHYL. But the other disciples began to be saddened on account of the word of the Lord; for although they were free from this passion, yet they trust Him who knows all hearts, rather than themselves. It goes on: And he answered and said to them, It is one of the twelve, that dips with me in the dish.
BEDE; That is, Judas, who when the others were sad and held back their hands, puts forth his hand with his Master into the dish. And because He had before said, One of you shall betray me, and yet the traitor perseveres in his evil, He accuses him more openly, without however pointing out his name.
PSEUDO-JEROME; Again, He says, One out of the twelve, as it were separate from them, for the wolf carries away from the flock the sheep which he has taken, and the sheep which quits the fold lies open to the bite of the wolf.
But Judas does not withdraw his foot from his traitorous design though once and again pointed at, wherefore his punishment is foretold, that the death denounced upon him might correct him, whom shame could not overcame; wherefore it goes on: The Son of man indeed goes, as it is written of him.
THEOPHYL. The word here used, goes, shows that the death of Christ was not forced but voluntary.
PSEUDO-JEROME; But because many do good, in the way that Judas did, without its profiting them, there follows: Woe to that man by whom the Son of man is betrayed! good were it for that man if he had never been born.
BEDE; Woe too to that man, today and for ever, who comes to the Lord’s table with an evil intent. For he, after the example of Judas, betrays the Lord, not indeed to Jewish sinners, but to his own sinning members. It goes on: Good were it for that man if he had never been born.
PSEUDO-JEROME; That is, hidden in his mother’s inmost womb, for it is better for a man not to exist than to exist for torments.
THEOPHYL. For as respects the end for which he was designed, it would have been better for him to have been born, if he had not been the betrayer, for God created him for good works; but after he had fallen into such dreadful wickedness, it would have been better for him never to have been born.