Jamieson, Fausset and Brown on Mark 9:42

42. For whosoever shall offend one of these little ones that believe in me--or, shall cause them to stumble; referring probably to the effect which such unsavory disputes as they had held would have upon the inquiring and hopeful who came in contact with them, leading to the belief that after all they were no better than others.

it is better for him that a millstone were hanged about his neck--The word here is simply "millstone," without expressing of which kind. But in Mt 18:6 it is the "ass-turned" kind, far heavier than the small hand-mill turned by female slaves, as in Lu 17:35. It is of course the same which is meant here.

and he were cast into the sea--meaning, that if by such a death that stumbling were prevented, and so its eternal consequences averted, it would be a happy thing for them. Here follows a striking verse in Mt 18:7, "Woe unto the world because of offences!" (There will be stumblings and falls and loss of souls enough from the world's treatment of disciples, without any addition from you: dreadful will be its doom in consequence; see that ye share not in it). "For it must needs be that offences come; but woe to that man by whom the offence cometh!" (The struggle between light and darkness will inevitably cause stumblings, but not less guilty is he who wilfully makes any to stumble).