There are gender expectations in play a bit here I think. Women are supposed to be gentle, peacemaking caregivers and historically have been punished for acting against those expectations. (Which is another reason her ending up at a 60's Stepford farm is so perfect.) I suspect some of that is in play here even if not deliberately. Five is a man so killing and fighting is expected.
There is also a big difference in how they kill. Five acts deliberately and coldly and in a way we have conditioned to accept by media. We accept Five's I did what I had to do explanation because we have heard it so many times before. With Vanya her actions are much more passive and often accidental. Which makes her much more terrifying since who knows what will set her off. Her powers are tied to her emotions and trauma and unpacking that is complicated. Plus women's emotions are so often portrayed as a terrible mystery particularly in scifi and horror. So while Five and Vanya are both monsters Five is a much more familiar one (and often a type that is portrayed as a hero) so people are much more accepting of Five's monstrous acts.
I also think people just have different definitions of justice and redemption. Personally I think Vanya was well past the point of not being mentally capable of being responsible for her actions. She was essentially having a psychotic break, which doesn't make her a psychopath just human with a lot of trauma. (One of the weird effects of Sociopaths being trendy is people are quick to label anyone not completely normal as a sociopath before that they were bi-polar) A lot of Americans often seem to have this need for punishment that is probably the Puritan's fault somehow. I remember when a schizophrenic beheaded someone on a Greyhound bus a few hundred kliks from here were a lot of stories from the US that were absolutely appalled that the man wasn't jailed for life. To me (and most Canadian's) locking up someone with a treatable illness seems like a waste of time and money. I feel like Vanya is a similar case any punishment would have just been piling on to an already plot heavy and traumatized character.