"He understands that, given the opportunity, the SNP will wreak havoc and spell disaster."
Why would the SNP do this?
They run a good government up here, they ran a Minority government up here for 4 years ,done well enough to gain an outright Majority at last election And are still getting more popular. This doesn't seem to happen in UK politics very often.
As For tactical voting, if the main parties actually gave a crap they wouldn't Have to ask voters to vote for Some one they dOnt want, just to save their butts