Albert Barnes on Matthew 21:44

Whosoever shall fall, etc. There is an allusion here, doubtless, to Isa 8:14,15. Having made an allusion to himself as a Stone, or a Rock, Mt 21:42, he proceeds to state the consequences of coming in contact with it. He that falls upon it, shall be broken; he that runs against it--a corner-stone, standing out from the other parts of the foundation--shall be injured, or broken in his limbs or body. He that is offended with my being the foundation, or that opposes me, shall, by the act, injure himself; make himself miserable by so doing, even were there nothing farther, But there is something farther.

On whomsoever it shall fall, it will grind him to powder. That is, in the original, will reduce him to dust, so that it may be scattered by the winds. There is an allusion here, doubtless, to the custom of stoning as a punishment among the Jews. A scaffold was erected, twice the height of the man to be stoned. Standing on its edge, he was violently struck off by one of the witnesses; if he died by the blow and the fall, nothing farther was done; if not, a heavy stone was thrown down on him, which at once killed him. So the Saviour speaks of the falling of the stone on his enemies. They who oppose him, reject him, and continue impenitent, shall be crushed by him in the day of judgment, and perish for ever.

(g) "it will grind" Heb 2:2,3