Fully understand your feelings towards the nationalist element of the SNP.
However, from a Scottish point of view I believe the reasons so many voters up here have turned to the SNP is because Labour has drifted too far from its traditional Left-wing socialist roots; which is what made it the go-to party for the majority of Scottish voters for so many years. Scot's aren't necessarily voting for the SNP in this election to show they back the separatist policies of the SNP (I believe a recent referendum also backs this opinion), they just want a stronger left-wing Scottish representation at Westminster level. In an ideal world we would be voting for the SP (Scottish Party), pretty much the same as the SNP but without all the separatist bits.
Tories are a write off in Scotland, so they don't even equate.
Lib Dems, whilst they have some sound policies and are a bit more center, they are never going to form the core of a government so whats the point in voting for them.
Greens, good for the environment; not much good for anything else (we'll probably all be voting green in 50 years anyway when the oil has ran out and there's no food left)
UKIP, LOL.
So whats left?
Obviously I cant't speak for everyone in Scotland - but what I've outlined above is how that majority of people in my circle of friends and family feel about the political landscape up here, particularly since the referendum.