I am genuinely struggling to understand.
If there is a minority Labour government, the SNP will presumably vote with that government in a lot of the more "left-wing" votes, with or without any deal. Raising the top rate of tax, for example, or introducing a mansion tax. It is my understanding that they support these policies anyway.
The SNP will presumably vote against the government on issues like the renewal of Trident. However, unless they choose to play a very dodgy political game, the Tories will surely vote with the government on such issues, so the SNP will be irrelevant.
What issues would fail to attract support from either side of the house, thereby forcing the Labour Party to enter into a potentially damaging deal with the SNP?