John Chrysostom

Intolerable sorrow thereupon seized that holy company. And John indeed saith, they "were in doubt, and looked one upon another," and each of them asked in fear concerning himself, although conscious to themselves of no such thing. But this evangelist saith, that "being exceeding sorrowful, they began every one of them to say unto Him, Is it I, Lord?" Mark at what time He discovered him. …

Homily on the Gospel of Matthew 81 ↗

Theophylact of Ohrid

The other disciples were agonized, for though the conscience of each was clear, they put more trust in Christ than in themselves since He knew their hearts better than they did.

Commentary on Matthew ↗

Jerome

And greatly saddened, they began to each say: Am I the one, Lord? And certainly the eleven apostles knew that they had not thought anything like that against the Lord: but they believed more in their teacher than in themselves, fearing their own weakness, they sadly asked about the sin whose consciousness they did not have.

Commentary on Matthew ↗

Thomas Aquinas

Then follows the effect, and it is a twofold effect: sorrow and doubt. As to the sorrow, he says, and they were very much troubled. And why? They were troubled at the death of Christ, because it was bitter for them to be deprived of such a leader, such a patron. Likewise, they were saddened at such a crime that was about to happen; Jeremiah 9:1: who will give water to my eyes, a fountain of tears? Then the doubt is set forth: they began every one to say. But why did they doubt? Was not each one certain about himself? …

Commentary on Matthew ↗