What is the government's moral duty?

In an article laced with dodgy statistics on the progress of spending cuts so far, not to mention the effects of tax cuts in this parliament, the prime minister suggests that governments have a moral duty to lower taxes.

Is that right?

I would have thought that governments have a moral duty to ensure that taxation is fairly levied so that the government's primary moral duty to support the weakest and most vulnerable in our society has sufficient funds.

That would suggest that a government's moral duty is to progressively tax everyone to ensure that it has enough money for the services for which it is responsible and a measure of redistribution (always a feature of the tax system).

Such would be view of both Warren Buffett and Bill Gates who, last time I looked, were not people of the extreme left.