All the opposition leaders and a large number of their MPs have to take some blame for Brexit for supporting a general election, whilst trailing hugely in the polls and getting absolutely nothing in return. The SNP supported it purely to shore up their own, one trick naked ambition. The Liberals have no idea why they supported it unless Swinson was actually suffering from delusions and thought she could be PM, whilst Corbyn supported it for god knows what reason, unless he was a similarly deluded as Swinson. In many ways it is probably the best outcome that none of these charlatans will be negotiating anything as they appear incapable of knowing when they hold the whip hand!
Johnson was desperate for an election and they gave it to him, for free. Not only that, by securing nothing in return, they let him have home advatage and a 5 goal start. Whilst Johnson will forever be associated with Brexit, if there are any negative consequences that flow from it, all the opposition parties need to recognise the roll they have played in facilitating it.
Should they have insisted on a second referendum, whilst being clear that the 'question' was going to be Johnsons deal or remain, he would have found it quite difficult to refuse, given how 'brilliant' he has continually said his deal was.