Lions are surprisingly not vicious towards humans at all--since there's no species that can fuck with them they don't have the vicious fear response most animals have. That's the real trick to lion taming--and the reason you can tame a lion but not a jaguar. Tigers are the same way. The big danger with either of them is actually playfulness--they don't always realize how strong they are.
“Dave Hoover:
“The chair has four legs... Now, an animal has a one track mind, For instance, the animal is coming after you with the idea of tearing your head off... You put the chair up, and all of a sudden, he has four points of interest. He loses his original train of thought because this agitates him. He can’t comprehend those four points of interest, so what he does he attacks the chair. He takes his wrath out on the chair. His mind now had been completely distracted from his original thought: ‘Eat the man in the white pants.’
“It’s basically animal psychology. You try to keep the animal afraid of you in that he does not understand you. He does not understand that you’re weaker than he is. [...]”
(The next few paragraphs are also quite interesting, and I recommend the film generally.)