Because he remembered not to show mercy, but persecuted the poor and needy man, that he might even slay the broken in heart.
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Augustine of Hippo
16. "And that, because he remembered not to act mercifully" (ver. 15); either Judas, or the people itself. But "remembered not" is better understood of the people: for if they slew Christ, they might well remember the deed in penitence, and act mercifully towards His members, whom they most perseveringly persecuted. For this reason he saith, "but persecuted the poor man and the beggar" (ver. 16). It may indeed be understood of Judas; for the Lord did not disdain to become poor, when He was rich, that we might be enriched by His poverty. But how shall I understand the word "beggar," save perhaps because He said to the Samaritan woman, "Give me to drink," and on the Cross He said, "I thirst." But as to what followeth, I do not see how it can be understood of our Head Himself, that is, the Saviour of His own body, whom Judas persecuted. For after saying, "He persecuted the poor man and the beggar:" he addeth, "and to slay," that is, "that he might slay Him," for some have so rendered it, "Him that was pricked at the heart." This expression is not commonly used except of the stings of past sins in the sorrows of penitence; as it is said of those who, when they had heard the Apostles after our Lord's ascension, were "pricked in heart," even they who had slain the Lord. ...