Answer and Explanation
Hume clamis in that passage, and this is the essentials of his theory, that ethics and morals are not determine by reason. That is why it is no unreasonable to think of one self when reason is in charge. There for selfish thoughts as the destruction fo the whole world are effects of reason, that is why he calls them not unreasonable.
Hume claims that for a person to acts according to ethics and morality, sentiments or afections must be placed. The point he wants to make is not about justifying selfish acts, but to point out how humans and their reasoning can be destructive. And to underline the importants of other matters in human life such as afection and feelings.
Hume undermine objectivity in ethics because objectivity comes from thinking and reasoning, which as we already explained is not ethical. To act in a moral and ethical way, is to think about the consequences of every action, and also to consider others.
To have an ethical thinking is very important for society, the impact our actions have is necessary.