Murdoch is responding in the third paragraph to issues around moral complexity, such as the difference between duty and virtue. Murdoch associates "duty" with "orderliness"; in a morally ambiguous world, rules are needed to preserve humans from self destruction. They provide a much-needed simplification. She talks about different moral "styles," which we must navigate between as we balance different obligations. They cause one to "have to choose between being two different kinds of person." Murdoch's discussion here of how the pursuit of virtue can lead to different "selves" is very compelling.

Tempting though it may be to use religious precepts or "duty" to simplify moral conduct, Murdoch argues that there should be "time off" from the demands of duty, but "no time off from the demands of good." She admits that the decline of religious absolutism in the West has resulted in an increase of happiness, albeit at the cost of moral certainty.

I can't speak to your own opinion of these matters, but Murdoch's writing here suggests a particular conception of the individual as someone who naturally tends to free thought. That is, the choice to embrace a moral code is an individual one; because it is a choice, even the decision to adhere to "rules" is a kind of moral relativism.