A beautiful and sexually free woman, Helene becomes Pierre's wife, then cheats on him and falls deeper and deeper into a morally questionable lifestyle.
OK, before we go too far down the road of Tolstoy's misogyny, we do have to kind of hand it to him for actually creating two female characters who are into sex: Natasha and Helene. Because they are Pierre's two love interests, they form a nice little compare/contrast example for the reader. All in all, because Natasha and Pierre end up so clearly physically hot for each other, it's hard to write Helene off as yet another loose-woman-comes-to-bad-end character (a character type that litters the pages of many a 19th century novel).
Yes, that's basically what Helene is, but there's at least a little nuance there. It isn't her desire that does her in, but the fact that she takes her physicality to a creepy and gross level. Need some examples? No problem:
Helene is actually a pretty outrageous character, and not just for Tolstoy's time. Think about every femme fatale, and there's a bit of Helene in each of them.