You don't need psychopaths to get that. And of the people actually willing to make taboo tradeoffs, take risks and hurt or manipulate people it isn't the psychopaths who are most likely to do so in a way that benefits 'society' as opposed to themselves at the expense of society.

Just for the sake of argument (see "recombination" above), you might still need psychopaths for there to be a sufficient amount of favorable genes in the pool so that normal people can get nasty in the altruistic way.