Probably because mankind invented tools and weapons which can put any other animal in danger. An unarmed man may not be any more dangerous than a cat, but an armed and equipped man can potentially harm any other animal.
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The Most Dangerous Games
The things that have killed the most of our population, aside from old age, are other people; we are the most dangerous entity to ourselves.
Highwaymen that were the worst and most dangerous and came from Sussex.
Hal Wulff is the most adventurous man on the planet hands down.
Alexander Hamilton