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Maybe have their first encounter go as such. They don't know each other. But the villain might kill someone the hero has a personal attachment to, and so it gets personal next time. Good luck!
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/24717 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/