It is often said, "The best defense is a good offense", which tends to work if you aren't already in a corner. So if we flip the saying on its head and always play defensively, you would probably never get anywhere, because you are only responding to attack toward you -- which might be what some people want.
So what if you extend this to human interaction, which can often be considered one more front of war. If a person plays defensively by default, it can often be perceived as being, or even playing, the victim. In Sengoku Nadeko case, these attributes can quite clearly be place on her, because she both acts and is often thought of as a victim. So with the end(ish) of Otorimonogatari, we see her in a new light that isn't going to be very favorable to some.
The way Otorimonogatari started was an interesting one that served as a way to peek people's interest and point out the one thing people jump to when they think a character is innocent. Without any context, you would guess that the snake god was the cause of Nadeko's Medusa-like appearance -- this is where both Nadeko's body language and image plays an important factor. Because of these factors, more viewer are fooled into thinking that someone like Nadeko would never be as evil as the oddity she became; in reality, all of the sweetness is a big farce that fooled many people -- mostly males apparently.
The main reason for even going to such great lengths to hide her true personality is a matter of love for a non-confrontational life; Nadeko used several tools in her arsenal to protect herself from the many inconvenient things in her life. One of them being more of an outer attribute, that being Nadeko's bangs, which were used to cover her face. By simply being timid, there weren't many people who would interact with her for too long, since they were unsure how she felt.
When her bands got cut, one of her main defenses was broken and Nadeko had no other choice but to expose her face -- both physically and mentally; and by this point we could piece together that most of the personality "changes" were on her accord. The sweet and innocent victim that Nadeko painted for herself starts to chip away as her real, more aggressive, personality manifests.
And the idea of deception really does come full circle when we find out that not only is Nadeko fooling others, she is also fooling herself. Since the snake Nadeko was interacting with was a dead god, all of the conversation she had -- up until she swallowed the talisman -- with the snake was her inner monologue, presented in a different manner. The end goal for Nadeko was to get the talisman that Koyomi had in his possession, so she made up a story and believe it all the way until the end. So in a sense, she got so caught up in her own delusions, it ended up swallowing her whole; and all that was left is her jealousy, anger, envy, etc. for the broken idolized crush she had over Araragi Koyomi.
I don't normally like shy girls in stories, but the way the Monogatari Series presented Nadeko, was good enough to make me want to learn more about this character. She ends up transitioning from victim to aggressor in a matter of four episodes, which ends in the assumed tragedy of killing off three major characters, all because of Nadeko not being able to control her unkempt emotions.
I wouldn't say that the imagery of her Medusa appearance makes much parallels in both stories, but the idea of Nadeko becoming ugly because of her insecurities is something that mirrors the Medusa myth. Also, I don't feel sorry, nor do I dislike, Nadeko for what she has become. Not everything ends happily and sometimes victimhood isn't always going to mean intrinsically good. If she wanted help she could have got it, but help can only go so far if you aren't willing to help yourself.