Okay, so the first paragraph. I am not talking about life or death situations, we are talking about Machiavellianism here and not psychopathy. This is along the lines of if you were given the decision to give yourself a promotion, an all around better scenario, you would chose yourself over your peer. In general people think themselves better than everyone else, if they aren't it isn't by that much or it doesn't matter. So it is natural that they chose themselves over others if there is no apparent gain in helping.

Good example would be giving someone $100 if they don't help an old lady up from the ground. Depending on the person they may chose either or, but both of which are self interested. We help people because we feel pain due to empathy and feel good when they are happy. So in that sense things like charity are acts of selfishness because they serve the person that is doing it. The money in this scenario is to show a physical incentive and something more cerebral are the same.

Now your second paragraph. Something is useful when it is line with your goals. Like a shovel for digging holes or a fork for food. People are the same but they have to be bent and directed in the way you want in order to be useful. For example lets say that you and your wife are going out to meet some big wigs at your company, people who weld a lot of power and can change your current situation with a snap of their fingers. Yet your wife is in the slums. She feels fat, shes tired, nothing looks right, nothing fits right, etc. To have an optimal impact you need to function as a team, you can't have dead weight. So you encourage her, and do so with your words, body language, and inflection of tone. Let's say that it was successful, she is more useful than she was before in this situation for your specific goals.

Also no one even thought that you were stupid or rude before you mentioned it.