I find that assuming that whatever rotten thing people can do, they eventually will do, is the sound basis for a political worldview. And this is why I am a conservative, not a liberal or a libertarian.
I concur with Rod Dreher. The bottom line difference for conservatives as compared to many liberals and libertarians has to do with the perfectibility of man. Now, don’t take this as a blanket statement. Those are generally unhelpful and inaccurate. Even still, I’d guess this can be fairly useful in helping people understand the skepticism (if not outright cynicism) that comes from conservative folk like myself.