Three main stands of ethical thought

Consequentialism

is the ethical theory that people should choose their actions based on the outcomes they expect will result. How to judge outcomes is not specified, but there are many types of consequentialism that specify how outcomes should be judged.

Utilitarianism

For example, utilitarianism holds that the best outcome is that which maximizes the total welfare of all people, and ethical egoism holds that the best outcome is that which maximizes their own personal interests. from Less Wrong

Utilitarianisms problems

• average utilitarianism evaluates the existence of a single person with 100 hedons more favorably than an outcome in which a million people have an average utility of 99 hedons
• Average utilitarianism seems to reject what Parfit calls "mere addition": the addition or creation of new lives that, although they may not be as happy as the average (and thus bring down the average), may still be intuitively well worth living. Creating a less-than-average life would become an immoral act. Furthermore, in a world where everyone was experiencing very bad lives that were not worth living, adding more people whose lives were also not worth living, but were less unpleasant than the lives of those who already existed, would raise the average, and appear to be a moral duty.

Deontology

holds that people should choose actions which conform to a prescribed list of moral rules,

Virtue ethics

holds that people should be judged by how virtuous they are, instead of by what actions they take.