I want to write a essay about business ethics. Topic is that explain why a good person does a bad thing. Please give me some ideas

I would suggest creating a scenario in your paper of a person who starts in a job with the best of intentions, and then is put by others such as supervisors into a position in which there really is not a good choice available. What ideas have you come up with?

You mentioned business ethics, but you might want to look into some of the psychology research. It was a while back, but I remember something from my intro psych class, I think it was called group think effect. Basically, when people get together in a group, if one or two people come up with a bad idea and push it, the rest of the group might tend to go along with it just to avoid making waves even if they know it's a bad idea. We saw a whole video on it, th scenario was kids working on a group project but one of the real life examples of group think and negative effects was one of the space missions that went wrong (I forget the details, but I'm sure you can find it easily online).