Theophylact of Ohrid

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 unto them, It is one of the twelve, that dippeth with me in the dish.

Catena Aurea by Aquinas ↗

Bede

But they began to be sorrowful, and to say to him one by one, "Is it I?" And surely the eleven apostles knew that they were not contemplating anything like this against the Lord, but they trusted the teacher more than themselves; and fearing their own weakness, they sorrowfully inquired about a sin of which they had no consciousness.

On the Gospel of Mark ↗