Why Labour lost

The accepted narrative about why Labour did so badly in the General Election seems to be that they failed to appeal to the aspirations of the 'squeezed middle' or to inspire confidence in their ability to manage the economy. I am not convinced by this. I have no solid evidence so my own 'analysis' is purely anecdotal and based largely on conversations (both participated in and overheard) with friends and work colleagues.
I believe that in the last week or so before the vote, scare tactics employed by the Conservative Party and the media around the possibility of a coalition between Labour and the SNP began to have the effect of convincing 'undecideds' that a vote for Labour would be a bad idea (I certainly heard people express the view that the SNP 'only care about Scotland' and that they would somehow use any power they might have in government to benefit Scotland at the expense of the rest of the UK) and, realising that it was working, the purveyors of this narrative concentrated very hard on getting their message out as powerfully as possible, particularly via the tabloid press.
I believe Ed Miliband's response to this development was mistaken and actually exacerbated the effect of the scare. What he did was to make increasingly shrill declarations of his determination never to do any deals with the SNP, which I believe had three (inter-related) negative effects. Firstly it confirmed in people's minds that the SNP were to be feared, secondly it was very difficult to believe since the prevailing view was that Labour would not win enough seats to be in a position to govern alone and thirdly, if he was telling the truth then what was the point in voting for a Labour party that wasn't prepared to do what was apparently going to be necessary in order to form a viable government and keep the Tories out of power?
In my opinion, Miliband should have embraced the idea of an anti-Tory coalition involving Labour as the dominant partner with support from the SNP and others and made the case for the SNP as having a valuable contribution to make to the politics of the UK as a whole. This could have taken the sting out of the Tory and right-wing media scare tactics.
When asked by Jeremy Paxman, earlier in the campaign, whether he was tough enough to stand up to the likes of Vladimir Putin, Ed Miliband's response was a strident "Hell, yes!" Surely, then, he should have been able to convince middle England's swing voters that he was strong enough to be in coalition without being dominated or manipulated by Nicola Sturgeon or any other potential coalition partner.