By "dangerous" I mean that it is a venomous pit viper. Personally, I have never met an actually aggressive snake, though I've encountered hundreds in the wild. The only venomous ones I've been able to meet in the wild so far are all pit vipers : Copperhead, Water Moccasin, & Pygmy Rattlesnake

A defensive venomous snake can be dangerous, so I always try to be careful.