Arguably the Conservatives have the SNP to thank for the seats they kept/picked up, and the possibility of a majority in parliament by a whisker.
Mrs Sturgeon should have known that by invoking a Scottish nationalist call to arms to inflict such a humiliating defeat on Labour in Scotland, it would damage Labour elsewhere, invoking nationalism elsewhere.
In particular her continual statement that under NO circumstances the SNP could work with a legitimate largest party Conservatives, which clearly was her democratic right, but hardly fair or 'British'.
England saw a threat on the prospects that a 1970's early 1980's SNP throwback, insisting we followed the 'spend, spend, spend' mistakes of the 2000's - would block every Conservative policy rather than look at the merits of those policies for the national good - producing a 'lame duck' parliament for 5-years at best.
Not helped if they heard, Miliband in the event of a larger party Conservatives, had legal eagles going over the constitution to work out how to EJECT Cameron and install himself as P.M. based on that anti Conservative SNP promise.
Adding a constitutional mess, to the ones they left in 2010, and showing a power at any cost, 'grubby' side.
Labour paid for that and a bland, badly thought out, populist, soundbite of a manifesto insisting it covered all - while uttering class/wealth division in every speech - for a 'better Britain', and many people would have seen through that.