Humans are probably the most unpredictable beings the crows have seen.
How would they ever know you're not a threat for sure? Usually, it's hard for a human to kill another (unless with a lethal weapon and lots of intention), but it's really easy for a human to kill a bird, even recklessly or carelessly.
Note that humans in developed societies are generally much "nicer" than what we have been for thousands of years. And even so, for many individuals today, they're not different from our barbaric ancestors...
How would they ever know you're not a threat for sure? Usually, it's hard for a human to kill another (unless with a lethal weapon and lots of intention), but it's really easy for a human to kill a bird, even recklessly or carelessly.
Note that humans in developed societies are generally much "nicer" than what we have been for thousands of years. And even so, for many individuals today, they're not different from our barbaric ancestors...