Black mambas are not the 'meanest'. No animal is mean, I don't care what you say. There are better terms like 'defensive'. Any animal used to predatation in the wild is gonna be scared of you! Anyway about mambas, not the meanest. BUT the most dangerous when provoked. You piss them off, they WILL kill you if they see fit. They are territorial, fast, and crazy venomous. But mean? No.
All those snakes you mentioned are usually wild caught animals. A captive-bred and well-handled ANYTHING is most likely going to be well behaved unless the handler makes a fault. (Which happens a LOT, and is blamed on the animal for being 'mean')
Let them do what they want, and respect it. If they let you handle them, be honored and enjoy it.