What does duty ethics (Pflichtenethik / deontology) evaluate, and what are its strengths and weaknesses?

It evaluates the action itself against a catalogue of norms saying what is required or forbidden — giving clear, unambiguous guidance, at the cost of rigidity when the rule produces a terrible result.

Duty ethics starts at the actions that are commanded or forbidden. There is, as it were, a catalogue of norms that states concretely which conduct is right and which is wrong: thou shalt not steal, thou shalt not kill. The duty to tell the truth holds even when the truth brings the individual a great disadvantage.

The advantage is real and often undersold: clear and unambiguous instructions for action exist. You know in advance what you may do; so does everyone else; and the rule cannot be argued away by whoever is best at constructing a justification. That predictability is what makes rights function as protections rather than as suggestions — a right that yields whenever the sums favour yielding is not a right.

The weakness is the mirror image. Faced with the consequences of unconditional compliance, one is forced to ask: must it not be permissible to relativise duties situationally? The murderer-at-the-door case is the standard pressure test, and any honest deontologist has to say something about it.

Note the relationship between the theory and its problem. Weakening the rules to handle hard cases dissolves exactly the property — unconditional bindingness — that made the theory attractive. This is why sophisticated versions do not simply soften the rules; they specify the duty more precisely, or arrange duties in a hierarchy, so that hard cases are handled by the rules rather than by making exceptions to them.

Go deeper:

• Stanford Encyclopedia: Deontological Ethics — agent-centred constraints, the doing/allowing distinction, and how the hard cases are handled from inside the theory.
• Wikipedia: Deontology — the overview and the non-Kantian versions (Ross's prima facie duties, contractualism).