The thing about the military – something Nikolai will appreciate after he loses all that money later in the book – is the structure it provides. There is no guesswork to figuring out what you are supposed to do, you just follow orders and all will be well. Mostly. And for the unimaginative. Yet in society while there are rules, it is a lot more ambiguous for someone who is weak willed like Pierre.
Tolstoy is concerned with the idea of a person having a moral center that influences all of their actions. People who do not have a center, such as Pierre, are easily manipulated, whereas someone like Hélène has a corrupted center and can only cause trouble. She is described always by her physical beauty followed by Pierre remarking how she is otherwise rotten. He is learning to judge people. Slowly.
Tolstoy implies Hélène had an incestuous relation with her brother Anatole.