You are forgetting the fact that the Tories scaremongered voters by claiming a Labour/SNP partnership would be bad for Britain, that won the Tory's their majority, they were banging that one home to the masses for 2 months.
Without the SNP, the conservatives would have been out.
Don't buy that. It's scapegoating.
The tories won because noone wanted Miliband as prime minister.