Gregory the Great on Jn 10:10, as reported by the great commentator, Cornelius a Lapide:
The thief and robber of the sheep, who does not enter the sheepfold of the Church through the door, that is, through Me [Christ], but breaks in secretly some other way, as for instance a heretic or schismatic, a scribe or Pharisee, or especially a false-Christ, who passes himself off as the Messias… comes not to protect and save the sheep, but to steal them and to snatch them from God and the Church, whose property they are, and to bring them into their own gathering, the synagogue of Satan, and there slaughter them by heresy and sin, and lead them to their destruction in hell.”
[By contrast, the good shepherd] drives away wolves, serpents, and everything that is harmful to the sheep; so let the pastor drive away all heretics and harmful persons. And let him feed his flock with doctrine and the sacraments, and not fatten himself on the milk and wool of his flock. Let him not be mercenary, seeking his own profit and honors….
In a time of tranquility very often the hireling, as well as the true shepherd, keeps watch over the flock. But the approach of the wolf shows the temper of mind with which each one was standing guard over the flock. The wolf attacks the sheep when the unjust man and the robber oppress those who are faithful and humble. But he who seemed to be a shepherd and was not, leaves the sheep and runs away, because he fears that some harm will come to himself from them, he does not venture to withstand their injustice…. He flies because he saw injustice and remained silent: He flies, because he conceals himself by silence….
The wolf, first, is the heretic; second, any wicked man, who strives to snatch and pervert the faithful by word or example….”
A Lapide comments: Hence, Christ leaves it to be gathered by contrast that the good shepherd, when he sees the wolf coming, neither flies nor forsakes his sheep, but stands up for them and fights even to death, and in this way lays down his life for them.
Would that a Lapide were read by many and all. You can purchase his 4 volume commentary on the Gospels, the best commentary available, in English through Amazon.