This is a pretty crappy analysis, but I don't want to think about this novel further than the first (or maybe sometimes the second) degree.
Part 1. The Underground (continued)
The groans (and the complaints?) that people do when they have a toothache is an expression of their pleasure, or else they wouldn't bother groaning about it. It's an expression of the pointlessness of a pain that nobody has inflicted on them, and of the realization that they are at the mercy of their teeth. They know the groans are useless and only serve to irritate everybody else solely for the groaner's pleasure.
(This reminds me of the people that ask for me to either (1) fornicate with their lives or (2) shove a red-hot poker up their life's arse, depending on which meaning of "fuck" you're working with)
So now the narrator talks about how, out of boredom, he would indulge in his fancy for the dramatic and work himself up into offense/remorse/rage, just because he was bored of the tedium of his life.
So people of action usually attack secondary causes when they take their revenge; they attack a symptom, and not its cause. Meanwhile, thinkers such as the narrator want to affect the true root of their problems, but every cause has itself a cause, and that one another, ad infinitum; since the primary cause can't be found, the thinking man can only seek revenge out of resentment, and not justice. And so, in all justice, affronts should be let go - to be, like a toothache, a pain without tangible cause.
Essentially, people who are defined by one of their negative traits have the boon of being identifiable.
Even if being virtuous is in one's self-interest, it can't be enough to guarantee that they WILL be virtuous; after all, there are countless people acting AGAINST their own best interests to follow their convictions. Also, self-interest is almost impossible to define, much less measure and give weight and compare in a rational manner.
Logically, civilization should make people milder, less bloodthirsty, and less addicted to warfare; but in reality, the civilized world is bloodier than ever. "Before, he saw justice in bloodshed and massacred, if he had to, with a quiet conscience; now, although we consider bloodshed an abomination, we engage in it more than ever."
To conclude, the idea that human behavior can someday be perfectly predicted through a complete understanding of the "laws of nature" is deemed to be bullshit, since the narrator predicts that out of the boredom of knowing all the problems that are predicted to happen, human ingenuity will get productive and start popping out new complications to life. How nice.