Very often comments will get downvoted on HN not because they are trolling, but because they are simply wrong, off-topic or not a popular point of view.
Yeah, that's a good point. I think you might have better luck viewing any heavily down-voted comments in popular threads.
I think HackerNews is rather light on trolling behavior, relatively speaking, a more generalized site like Reddit would probably prove a better sample-set of data for something like this.