It is advisable to get into a discussion about this question only after one knows what ‘supererogation’ means and how it has been discussed for centuries. In the Greek ethics (especially as Aristotle formulated it), the notion plays a very crucial role without it being related to a normative sense of duty. With the Catholics, especially as Aquinas gave expression to it, a normative notion of duty cohabits the moral universe together with supererogation. To the Protestants, it makes sense to speak only of duty and not of supererogation because the latter does not simply exist. Without understanding the issues involved in these controversies, to make a bald assertion about supererogation (while appealing to a Protestant commonsense idea of duty without even realizing its Protestant origin) is to “…rush in, where Angels fear to tread”.
For more, check “An expanded draft on Terrorism”