Disclaimer: I don't own Doctor Who.
River Song was raised to kill one man. From a young age, she was told that "The Doctor" would destroy worlds, kill innocent people and then simply fly off in his blue box to the next place he was bound to destroy. He took her parents, and was trying to find her (to kill her, obviously.)
Her nannies would share stories of The Doctor's greatness, and she wanted to believe them, that he was good, that he was merely trying to protect people and sometimes things happened.
When she met him, she realized he did destroy worlds, and didn't care.
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