Casuistry is a word that I do not really understand, as it seems both an insult and an adjective of praise (but probably not both). One meaning of casuistry is the notion that one would try to use any kind of reasoning in order to have a particular agenda or line of action to be justified. This is the kind of legal reasoning one would use to argue for something but not have the epistemic space to be convinced otherwise. This seems somewhat dangerous a mindset; to already agree and be inflexible upon a topic of argument: why use the appearance of argument at all?
Casuistry by contrast, can be undesrstood as the application of a universal to appropriate an ethical principle in a particular situation. In this case, we may consider ethical maxims, such as the principle of double effect, or the golden rule: as we consider them general rules, we may also maintain that they have wide applicability.