I'm not suggesting that the SNP would side with the Tories. The best result for SNP is a Tory led government, be it on their own (unlikely as that may be) or propped up by UKIP, the DUP, or the Lib Dems again.
http://www.theweek.co.uk/general-election-2015/63195/snp-s-big-secret-what-they-really-want-is-a-tory-win