I think also that those who rely on the "evil Tories" line or something like it don't realise that it is counterproductive. Sure, it talks to the base and it's got emotional appeal and fight the good fight and all that, and it doesn't require all that much actual thinking.
But the argument falls to pieces when people meet actual real-life non-evil Tories, so it tends to ghettoise support.
For me personally it's the last election and the £8 billion of uncosted welfare cuts. And the fact that while the Tories immediately began backpedalling on a whole load of other campaign promises, that was one of the few lines in the sand they wouldn't go back across.
I have sympathy for people who swallowed Cameron's idea of a modern, fairly liberal, business-friendly (European) Tory party, and I'd completely understand the sort of person for whom Corbyn is just too left-wing, but... basically people try to get out of it and say "well, it was the least worst option" or "I couldn't vote for Miliband", and that isn't good enough for me. If you vote FOR a party you should be able to justify that.
This isn't personally targeted at you, by the way, I'm just putting out there why people slip into it.