You're making the assumption that what politicians say in public is the same as they are thinking in private. The best outcome for the SNP, in order to accelerate their separatist agenda, is for there to be another Tory government (regardless of how likely or unlikely that is to happen). Their decades long mantra is: "we've got an evil Tory government that Scotland didn't vote for'. A Labour government supported by the SNP would mean losing one of their aces in their goal to secure another referendum. Its no coincidence that last year's referendum happened at the height of a Tory government in the middle of a difficult economic period.
Of course they'll keep peddling the " we'll support labour to keep the nasty Tories out" party line during the election campaign but then watch them conveniently blame labour when the post election cooperation talks break down, paving the way for a minority Tory government.
IMHO of course