Feel free to answer in the way that best describes your opinion. There isn't a single best way to answer this question. Just remember to support your opinion.
I'm going to answer this question by saying that the banker is the main character.
Yes, I would have behaved differently than the banker. First, I would not have made the bet in the first place; however, I'll pretend that I was willing to bet money on the topic. For sure I wouldn't have made the bet with that much money to risk. Rich men don't become rich and stay rich by being cavalier with their money.
Second, had I made the bet, I would have made a bet where I stood to gain something. If the lawyer wins the bet, he gets a lot of money. If the banker wins the bet, he gets nothing. The banker can't even claim that he was right. The topic of discussion that night was life in prison. The bet only succeeded in placing the lawyer in confinement for a certain amount of time. So even if the lawyer had quit after three years or thirty years, the banker is not proven correct that life in prison is horrible. It was never tested.
Third, I would not have considered killing the lawyer. I have never been in that exact situation, so I can't claim for sure that I wouldn't have thought about murder in order to keep my money, but I like to think that my moral compass is stronger than that. I don't like losing, but I will admit defeat as needed. The banker wasn't willing to do that.