If all the Eurosceptics had voted for May's deal, it still wouldn't have had the numbers to pass. I think its fairer to lay the blame on the opposition parties who voted against it largely to try to create chaos which they thought would lead them to an election they could win.
Otherwise you don't have an opposition, and then you don't have much of a democracy.
It's the job of the government to secure the votes it needs for its policies, and conversely to propose policies for which it can secure the necessary votes.
That's democracy.
To the contrary, May’s government only tried to appease the hard-liners in her own party. Johnson has taken it even further in the same direction.
Like, the first vote definitely wouldn't have passed with them, but the third definitely would have.
Now personally, I think the Opposition should have voted for the deal, but the primary respionsibility falls on the government.
May’s majority including the DUP at the time was 7 (I think - this was after the defections to the Independent Group) so those 6 would have tipped it into a loss on their own.