So, what's the general consensus on why Lester said "Yes," to Malvo in the elevator?
When prompted with the question in the first episode, Lester was completely confused and never seemed to entirely understand what he was agreeing to. But in Vegas, Lester obviously was well aware of what Malvo was capable of, and Malvo was making it clear that he was prepared to do something drastic-- "Is this what you want? Yes or no?"
Lester assuredly says "Yes." I've been thinking on why this would be. And I feel like the most common perception among viewers would be that he's simply gotten too full of himself and forgotten he isn't someone like Malvo and was just demanding respect. But, I feel like there's another layer to it. Specifically these two ideas:
1. Lester wanted to gamble. The last time he said "Yes," Lester lost his nagging wife, shut up his demeaning brother, became successful, etc. He seems to have rolled the dice and won, and now he wants to double down. The fact that they're actually in Las Vegas might highlight this intent.
2. As Malvo does what he does for personal satisfaction, Lester has simply come to enjoy the thrill of these situations, and would gladly take more at the expense of everyone around him. Despite his outwardly cowardly nature, subconsciously he's getting off on the danger. He's had a year of a happy relationship and a successful business. Now he's bored and wants a change of pace again.