"For all have sinned and fall short of the glory of God..."
and
"all our righteousness acts are as filthy rags..."
Thus the way of the hypocrite (i.e. all of us).
Why is Stannis so bloody righteous, anyway? Well, he had a whoring, spend-the-kingdom lummox of an older brother (who probably beat him up on a regular basis), and a bookend younger brother in Renly, except at the other end of the spectrum. Small wonder that Stannis is a Spartan.
The trouble with the guy who tries to be righteous, and also rams it down others' throats, is that he is, at some point, really going to screw up. The end effect of all of this is that the screw up is made that much bigger by having been preceded by all that holier-than-thou crap. Jimmy Swaggart, anyone?
Then there are the guys who try to be righteous, do a pretty good job of it, but when they inevitably mess up, they take it on the chin and do their best to clean up their mess. Ned Stark is a perfect example of what Stannis should have tried to be. It is Stannis' rigidity that will be his downfall.