Karin was not dumb.
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She was a lot of things (or so Suigetsu told her).
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But not dumb.
She knew that Sasuke was using them to gain power. He would probably throw Hebi away, like the "useless" trash he said they were. Sasuke was power hungry and Karin could see that, but as smart as she was, not even her could stop him. She saw him change over the months. At first his eyes were brighter, more happier. He seemed to like the company of Hebi, with her and Suigetsu always fighting (it was actually quite humorous), and with Juugo always looking out for him, making sure he was safe and healthy. Sasuke was even seen cracking a small smile from his team's antics. He was a happier person then, his chakra was warmer.
But he had changed into a monster. Not the kind of "monster" like Juugo, because Juugo was always remorseful and kind, but couldn't control himself sometimes. He never wanted to hurt anyone, but Sasuke did. Karin first noticed the change after Sasuke learned about his brother's true intentions. He no longer cracked those small smiles. Karin shrugged it off thinking maybe Sasuke was in a bad mood, his brother was dead after all. She began to worry when Sasuke would talk about destroying Konaha and all of his friends and team members. When he talked his voice was cold and dark, he always sounded like he was plotting something, something evil. She was losing him to Madara. She tried to get him to stop, but it always wound up to a slap in the face that would leave a dark bruise the next day. She lost him. Lost him to the bloodthirsty monster that no one could stop. How the heck did the Kyuubi boy plan to stop him? Sasuke was no longer on this earth, his thoughts were in hell probably making friends with the devil. Maybe the devil was Madara? Who knew?
Even Suigetsu and Juugo could see the change. Suigetsu already knew that Sasuke was easily brainwashed, he was so enraged that anyone could persuade him. Suigetsu already had no hope for Sasuke. Juugo on the other hand didn't want to believe the change in Sasuke. Sasuke was his only hope after all. He kept denying it, Sasuke was not evil. But you can only go so far with denying things.
Karin could be obsessive at times.
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But never dumb.
She too had been denying Sasuke's change because she was blinded with her infatuation with him. She didn't want to see his hate, she didn't want to see him change, She loved him too much. Her "love" for him was also like a monster she guessed. A monster she had easily beat, unlike Sasuke.
But every evil person could last so long. Robbers never got away with stealing anything, killers never got away with hiding the evidence, secrets could only stay secrets for so long, and Sasuke could never reach his goal of destroying Konoha and still be alive at the end, no matter how much the Kyuubi boy wanted to save him, sometimes the Uchiha just went to far.
She knew that Hebi was just a reminder of his old team. He never loved them, he loved Sakura, Naruto, and his sensei. Never them.
Karin knew all of these things, even when she was denying it.
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She was not dumb.