What's one disturbing thing you found out about someone you know?

This guy that was around my age when I was in high school used to walk me home from school. I didn't like him or anything like that and I honestly found him quite annoying, turned out to be a murderer years later.

He started murdering elderly women which I can't say I'm totally shocked by because he did seem off but I never would have thought I would have been walking with a future murderer.

Quite disturbing and makes you think how often are you to meet a murderer?
What's one disturbing thing you found out about someone you know?
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