A thought: yes, Buffy is often about doing what's right, even when it's difficult (though it must be mentioned that the characters often don't do this).
But what seems to me to be a more interesting aspect is that the characters are engaged in a fight that they will never (in fact, can never) win. There will always be vampires, demons, and hellhounds in the Buffy-verse. There's always another hellmouth.
That seems to me to be a more profound way of thinking: instead of doing what's right when it's hard, it's doing what's right when everything is telling you that it makes no difference. When the battle nevers stops.