Theophylact of Ohrid

And indeed, for a person who causes offenses and obstacles, it is better that "a millstone be hung around his neck and he be cast into the sea" than that he should offend and lead astray "one of these little ones," that is, the faithful.

Commentary on Luke ↗

Bede

It is better for him that a millstone be placed around his neck and he be cast into the sea, than that he should cause one of these little ones to stumble. He speaks according to the custom of the province, by which this punishment was for greater crimes among the ancient Jews, that they be drowned with a stone tied to them in the depths. And indeed it is better for the innocent to end their bodily life with a punishment, however atrocious, than to deserve the eternal death of the soul by harming a brother. …

On the Gospel of Luke ↗

Bonaventure

Third, as regards the horribleness of the punishment, he adds: It is more profitable for him if a millstone were placed around his neck and he were cast into the sea. Bede: "He speaks according to the custom of the province, where this was the punishment for crimes, that one would be plunged into the deep with a stone tied to him." This, I say, he calls more profitable than that he should scandalize one of these little ones: because this punishment is far less than that which he deserves by scandalizing his neighbor. …

Commentary on Luke, Chapter 17 ↗