I think another way of looking at this is to ask the question 'Why Shouldn't I Vote Conservative?' (I know, bear with me).
A few likely suggestions:
- They're complete cunts - true but also applies to the Labour party
- They'll make people's lives worse - true but also applies to the Labour party
- I don't agree with their policies - I think you get the idea now
The logical answer with respect to the 'lesser of two evils' argument is 'because Labour are better' isn't it. Which is fine but it's not in itself an argument never to vote Tory, it implies that would be fine in different circumstances. And the lesser evil argument is based on 'Tories out' as it's core moral force, so it needs an argument of the type above to be reintroduced. So the question is, if those things or your version of them gives a moral duty not to vote Tory (which I have no problem with), why doesn't it give the same moral duty not to vote Labour?