To me, mambas look like any old colubrid. I can see why people think rattlesnakes look wicked, as they have those scales over their eyes that give them a scowl,
& they tend to put forth some "attitude" to complete the look. But if you knew them better (as I do), you might feel differently...it's just self-defense. I think
cobras put forth a meaner look, hissing & hooding to look bigger than they are, but try to remember that we're the real predators, we're big & scary to all of them.
Personally, I'd give a much wider berth to any cobra, because not only is their venom worse than most rattlesnakes (neurotoxic kills you faster) but they are also
much faster & may come at you or spit, whereas rattlesnakes just sit there as if saying "go away!"