The Steinbeck quote that Walter spouts, "All war is a symptom of man's failure as a thinking animal", is apt here. I'm seeing tons of blind hate towards Arvo and his groups for attacking Clementine's group, but little insight into why they did it. Despite having a stereotypical bald-headed tough guy among their ranks, I doubt these Russians are stereotypical villains. They're not as bad as Carver, and they might not even be as evil as the bandits that attacked Christa.
It's clear that Jane was excessive in her mistreatment of Arvo. Despite him mentioning that he has friends in the woods, Jane apparently saw nothing wrong with robbing a crippled man, manhandling him, calling him a liar to his face (probably because she felt only she had a sister worth protecting), ignoring his claims that his family was suffering just as much as he was, threatening to murder him, and disregarding both Clem and Arvo's protests to remain calm. Even if Clementine treated Arvo with humanity, the fact that Jane acted so brutishly to him left him with the impression that Clem's group was run by a bunch of selfish bandits. If he returned to his community with news about a group of intruders who rob anyone they meet, it seems only fair that they be robbed and left to die.
There's also the fact that Arvo doesn't at all seem like the type to survive on his own. He's skinny, myopic and crippled. It can be argued that the group keeps him around simply because he's a translator and they take advantage of his pitiful appearance (he points out that he's not in charge of the group), but the fact remains that a group of hardcore survivalists still take the energy and time to keep a physical weakling and his sister alive in the forest, an effort which Carver and Jane would never have bothered with. I suspect that he was sent alone to the observation deck in order to drop off the medicine there before the rest of the group could arrive (perhaps needing a warm place for their injured woman), not expecting any living person to be there. However, when he told his people about the group that robbed him, they instead bade their time in the forest and waited to ambush them.
I do think Arvo is being honest about having an injured woman to take care of - he doesn't seem like the type to lie with a gun in his face. I doubt it's the woman who accompanies him in the cliffhanger ending, unless she turns out to be a drug addict. Other people have already translated the dialogue of the Russians and revealed that while aggressive, they started to have second thoughts when finding out about the baby. They might have backed off had one of Clem's group not shot the baby's mother - keep in mind that only Clem and Kenny noticed that Rebecca was a zombie during the stand-off.
In short, the Russians may have been more amiable had Jane used her brain and not treated Arvo like shit. Instead, she gets to dodge the consequences while Clementine is left to deal with the anger of a survivalist community that could have helped them get to Wellington.