Situation: You work at a suicide hotline.
Nice guy: Signs up for the job because he's always felt badly for people who are in despair. Has also seen how much pain this can cause those left behind, as one of his friends died this way. Gets involved in the lives of those he talks to, on two occasions actually meeting them in person. Eventually gives up the work because he finds it too upsetting, and can't handle it.
Average guy: Does it because he likes to think of himself as a good person. Makes an honest attempt to help, but steers clear of personal involvement. Figures he has saved four lives so far -- which is way more than most people have. (In fact, he has partially saved two -- which is still more than most.) Mentions his job at every opportunity, and finds it is a great way for getting dates with a certain type of girl. This discovery compels him to mention it even more frequently.
Sociopath: Works there so he can savor others' misery. Occasionally subtly encourages people to commit suicide, telling them under the guise of sympathy that their lives really are bleak, and that he understands why they think they have nothing to live for. Secretly gloats about the two times he pushed people over the edge. Lords his job over people who don't do such public service work, aggressively asking why they feel don't have to help out. Loudly attributes the nice guy's quitting to the fact that he "doesn't care."
Situation: Your school is holding a fundraiser.
Nice guy: Helps out, anonymously contributing what he can afford.
Average guy: Asks what everyone else is giving, then grudgingly gives that. Has no interest in being involved.
Sociopath: Either gets really involved, getting on the committee and taking over, or, if he can't take over, mocks the proceedings from a distance. If takes over, is actually skillful at fundraising, knowing just how to manipulate others into giving money. Makes a big show of talking about how it's important to him to "give back" to the community, and garners whatever publicity he can for his public spirit. If he can't take over, he tells everyone how stupid they are to give since the school just wastes their money anyway.
Situation: You've just plowed into a cyclist on a deserted country road; he is badly injured and only semi-conscious.
Nice guy: Stops car, gets out, checks cyclist, doesn't think twice, phones 911 immediately. Is extremely upset, and panicked. Tries to comfort cyclist, tell him that help is on the way, but is not sure cyclist could hear him. Feels terrible for cyclist. Briefly wonders what the legal repercussions will be.
Average guy: Stops car, gets out, checks cyclist. The thought occurs to him that he could just leave and probably get away with it, but he doesn't seriously consider this. Phones 911 and tells cyclist he just did so. Feels badly for cyclist, wonders if there's something he should be doing before the ambulance arrives, but he's read about people who are sued for doing that kind of thing and decides against it. His thoughts soon stray to what this will mean to him: will I go to jail? Shit, I wish I hadn't had that drink, but after only one drink my blood alcohol must be under the legal limit. I wonder if they'll find out about this at work....Geez, the guy looks pretty bad.
Sociopath: Calmly looks around, sees no one else is there. He sees through the window that the cyclist is hurt badly. Wonders if the cyclist will be able to identify him. Briefly considers running him over again just to make sure he can't, but decides against it, realizing that could land him in real trouble. Then drives off, thinking, they'll never catch me. And if they do, I'll just say I hadn't realized I'd hit anyone. Works self up into a fit of anger thinking, what the hell was that asshole doing going for a goddamn bike ride at this hour? Sees a Burger King and thinks, hey, a Whopper would really hit the spot right now, so stops to get one. Wolfs it down, his appetite undiminished. Arrives looking perfectly unruffled for his job at the suicide hot line.