It is a simple matter, after the fact, to assess whether harm was done and accuse those who decided to act of having intended to create the outcome, or having failed to prevent it, of unethical behavior. While it may be entirely logical to conclude that the action caused harm, it cannot be taken for granted that this was the result of an ethical failure.
The notion of "ethical failure" is in contrast to the notion of "unethical behavior," in the same way that failing to act ethically is not to be equivocated with acting unethically - it is a false dichotomy. While it is not inconceivable that an individually would choose to act in an unethical manner, instances of unethical behavior are exceedingly rare, and it is often the result of poor analysis that one cannot identify the positive intention that precipitated negative consequences, and therefore assume the intentions were unethical.
Having considered the matter at some length, I have identified six causes of ethical failure. I don't expect that this is an exhaustive list, but it seems a good start:
1. Malicious Intent
The first cause of ethical failure as an intention to act in a way that causes harm. As noted above, this is a common accusation that seldom bears true, but the possibility that an individual would act with the sole intention of causing harm to others.
2. Ignorance
Ignorance is the cause of ethical failure when an action is undertaken without thoroughly examining the situation or considering the possible outcomes of the action. It seems to me that this is the most common cause of ethical failure, and the most difficult to address, given that human beings are not omniscient and cannot be expected to have perfect knowledge.
3. Indifference
Another cause of ethical failure is indifference, which primarily manifests itself as a disregard for the interests of third parties who may suffer harm as a result of the actions undertaken to generate a benefit for others. Indifference is differentiated from ignorance (above) in that the actor is aware that the action will cause harm, but proceeds regardless of this knowledge.
4. Prioritization Error
Errors of prioritization occur in situations in which an action has multiple impacts (benefits for some and damage to others) as well as opportunity costs (undertaking one action eliminates the possibility of undertaking others) and while the individual is neither ignorant of nor indifferent to the outcomes, an error is made in prioritization of the impacts.
5. Probability Error
In a situation in which outcomes are uncertain, and in which the actor has undertaken due diligence to identify the impact of an action, errors in the assessment of probability may occur. Generally, the potential for benefit is overestimated while the potential for harm is underestimated, and a decision to act is made upon a poor assessment of the probability of known outcomes.
6. Execution Error
Given that everything involved in the decision to act was attended, there remains the potential for errors to arise in the execution of a planned action. I am not entirely certain that this properly classified as a separate cause of ethical failure, or merely a temporal issue: if a decision to act is made, and new information is uncovered, is the insistence on pressing forward on the original plan a separate matter, or merely the discovery of an error at a later time (conditions have changed after the decision is made, but during the course of action).
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Again, this is unlikely to be a comprehensive list of the causes of ethical failure, and each of these items bears more intensive consideration, but it's a fair start at exploring the notion of ethical failure.