Of course the hate for her is well deserved.
First off, blaming "brainwashing" lets her off the hook far too easily. Patty Hearst tried the same trick in the 1970's and it didn't exactly work out well for her. Ironically, Patty spent more time in prisoner for her bank robberies than Nana does for her 10+ murders, which in itself is unfair - Nana gets away with far too much because she's a girl, instead of in spite of it.
Yes, she would be hated just as much if Nana was male (probably more so).
It should be noted that all Nana's murders were premeditated, on her own cognisance and with malice. Just because she was told to do so, doesn't mean she had to.
In addition to that, just because she may not have wanted to do kill anyone, she was certainly happy to do so (smiling when thinking about killing Mirichu as well as the "won't be shy in killing you" part). Nana is a person who would rather murder someone than think of any sort of alternative (as is the case later on).
Futher more, stating that she's a "child soldier" carries no weight - she's killing civilians, which if she was a soldier makes her actions even more odious.
The fact that people try to exonerate Nana because she was "mind controlled" doesn't hold much water considering she was fully aware of what she was doing; didn't need to; didn't bother querying anything and was fully cognisant during her pre-meditated murders; and she quite happily carried another one out, with no doubt more to come.
In addition, there is no reason why she couldn't have asked questions or even did her own reason about Talents and so forth.
I wasn't surprised that the anime didn't get a second season (if it wasn't just for boosting manga sales) because Nana is so unrelatable, unrelatable and pretty much evil personified. Even later on, she's totally dislikable, obnoxious character.
Considering she's supposed to be intelligent, you would have thought, at the very least, queries the morality, if not the legality and ethics of killing schoolchildren (let alone those she killed before she arrived at the island). She's fully aware of what she's doing, so it's all on her own head. She certainly deserves to be punished far longer than three years (that ends up around 3 months for every kid).
I wouldn't be surprised if Nana Hiiragi does enjoy killing people - she is always smiling happily when thinking about killing her victims.
Whilst she may say that she doesn't want to kill any more, later on - it certainly doesn't stop her (no doubt it would be the first thing she thinks of to solve problems, instead of anything else).
Hopefully, she won't have a happy ending (preferably meet a nasty end - with her own poison needs would be nicely ironic). Whilst she may have "changed" for dubious reasons she will have to end up killing people again at some point. Even though she's changed, she's still an insufferable, nasty little bitch. I've got very little sympathy for her, especially as she was sadistic killing everyone.
And yes, killing Nano led to more people suffering - all because of Nana (no idea why Nano should forgive her - obviously he forgot how Nana taunted him before he fell, although I do hear he did beat the crap out of her as well).
Hopefully she will pay some sort of price for her actions.
Whist Nanao killed more people than Nana, it should be noted that Nana was the cause. It was nice of him really to leave Nana alone, considering she had no compulsion about killing Nanao - he certainly would have had a good reason to seek revenge on her.
