Objection 1. It would seem that the sin of Christ's crucifiers was not the most grievous. Because the sin which has some excuse cannot be most grievous. But our Lord Himself excused the sin of His crucifiers when He said: "Father, forgive them: for they know not what they do" (Luke 23:34). Therefore theirs was not the most grievous sin.
Objection 2. Further, our Lord said to Pilate (John 19:11): "He that hath delivered Me to thee hath the greater sin." But it was Pilate who caused Christ to be crucified by his minions. Therefore the sin of Judas the traitor seems to be greater than that of those who crucified Him.
Objection 3. Further, according to the Philosopher (Ethic. v): "No one suffers injustice willingly"; and in the same place he adds: "Where no one suffers injustice, nobody works injustice." Consequently nobody wreaks injustice upon a willing subject. But Christ suffered willingly, as was shown above (1,2). Therefore those who crucified Christ did Him no injustice; and hence their sin was not the most grievous.
On the contrary, Chrysostom, commenting on the words, "Fill ye up, then, the measure of your fathers" (Matthew 23:32), says: "In very truth they exceeded the measure of their fathers; for these latter slew men, but they crucified God."
I answer that, As stated above (Article 5), the rulers of the Jews knew that He was the Christ: and if there was any ignorance in them, it was affected ignorance, which could not excuse them. Therefore their sin was the most grievous, both on account of the kind of sin, as well as from the malice of their will. The Jews also of the common order sinned most grievously as to the kind of their sin: yet in one respect their crime was lessened by reason of their ignorance. Hence Bede, commenting on Luke 23:34, "Father, forgive them, for they know not what they do," says: "He prays for them who know not what they are doing, as having the zeal of God, but not according to knowledge." But the sin of the Gentiles, by whose hands He was crucified, was much more excusable, since they had no knowledge of the Law.
Reply to Objection 1. As stated above, the excuse made by our Lord is not to be referred to the rulers among the Jews, but to the common people.
Reply to Objection 2. Judas did not deliver up Christ to Pilate, but to the chief priests who gave Him up to Pilate, according to John 18:35: "Thy own nation and the chief priests have delivered Thee up to me." But the sin of all these was greater than that of Pilate, who slew Christ from fear of Caesar; and even greater than the sin of the soldiers who crucified Him at the governor's bidding, not out of cupidity like Judas, nor from envy and hate like the chief priests.
Reply to Objection 3. Christ, indeed willed His Passion just as the Father willed it; yet He did not will the unjust action of the Jews. Consequently Christ's slayers are not excused of their injustice. Nevertheless, whoever slays a man not only does a wrong to the one slain, but likewise to God and to the State; just as he who kills himself, as the Philosopher says (Ethic. v). Hence it was that David condemned to death the man who "did not fear to lay hands upon the Lord's anointed," even though he (Saul) had requested it, as related in 2 Samuel 1:5-14.
Source: Summa Theologica (New Advent)