I think Western leaders in general like to be seen to have the moral high ground on their side, which in the world of international politics is an expensive and possibly unsustainable luxury. It really is the law of the jungle.
In some cases your opponents are people who will just do what they want or take what they want without any such crisis of conscience or need for explanation, and in other cases they are insane, pure and simple. If they sense that others are encumbered by considerations of morality or conscience, it's just a weakness to be exploited.
That's not to say Western Leaders are actually any less immoral or opportunist than any others, but they do seem to feel the need to cover it with some sort of 'greater good' explanation. For instance, Tony Blair tied himself in knots trying to justify invading Iraq.