Oh, but you do deserve a mark even if your question and my reply disturb the moral pieties.
If ethical egoism is a coherent moral theory - and it is among the options standardly discussed (if usually rejected) in ethics tertbooks - then human conduct should be based solely on self-interest. 'Self-interest' is likely to provoke a volley of shots here, so let's say that ethical egoism is the view that everybody ought to look out for her- or himself alone, or that everyone ought to concern him- or herself only with their own welfare (as conceived by them).
As an ethical egoist you could defend, 'No, because I need to be paid', with complete moral propriety and consistency.
There would be a cost to ethical egoism if you value love, friendship, comradeship, but there is no logical need to include these values in your
idea of your own welfare.
Ethical egoism is rejected by Kantians, most utilitarians, human rights theorists but how strong are their theories ? Aren't the journals, and questions on Philosophy Stack Exchange, standing proof that none of the relevant theories is free from objections just as strong as any that they pitch at ethical egoism?
But if one plumps for utilitarianism, ethical egoism and utililtarian can be squared on two assumption. (1) If we accept Jeremy Betham's dictum that each person is the best judge of their own interests, the maximisation of interests is more likely to be satisfied if each person aims purely for their own interest. After all, they (rather than some well-meaning other agent) best knows what it is. (2) Interests, even best judged, may of course conflict but this is an empirical and contingent point. Conditions are imaginable in which they do not. If they do not, and if each person is the best judge of their own interests, interests are likely, more likely than not, to be maximised by ethical egoism.
References : P. Facione, D. Scherer & T. Attig, 'Values and Society', NJ, 1978, esp. p. 45. D. Emmons, 'Refuting the Egoist', 'Personality', 50, 1969. 309-19. And not to forget an older classic, H. Sidgwick, 'Methods of Ethics', 7th ed., 1907 : Bk II, ch.1.