[diminoten]

In your example, Bob should have first asked you to agree that you wouldn't share his idea, or use his idea in a way which would harm Bob's ability to benefit from his idea.

In reality, this is what happens. Aside from the utilitarian argument, there is a moral argument to be had here. You agreed to not make use of Bob's idea, and then you did. You broke your word, and are responsible for the pain you caused Bob.

You might argue that there are scenarios in which you never actually talk to Bob - Bob shows his idea to Tom, and Tom is the one who breaks his word. You're merely getting the idea from Tom, and are as such free from any moral obligation.

However, I would offer you this rhetorical in response: If you are offered goods in such a way that you reasonably know these goods to be gained through deceit or other immoral acts, is your acceptance of those goods (and therefore benefit of the immoral act) a moral act itself?