No matter what the definition of "right" is or which normative ethical model we use, killing a human being who poses great threat to the security of society is justified.

To demonstrate my argument, it is necessary for me to approach this topic from the two most common normative ethical theories: deontological ethics and utilitarianism.

Deontological ethics states that, under whatever circumstances or regardless of the consequences of an action, we must always abide by the categorical imperative — a set of moral imperatives that must be obeyed at all time. Among the many imperatives is to not kill a person. However, another element in the Kantian morality is what defines a "person". In his A Groundwork for the Metaphysics of Morals, Kant writes that a person is a being capable of reason, possesses free will, and abides by the categorical imperative. In other words, if a person does not abide by morality, he is no longer seen as a "person" in Kant's eyes, so killing him would be justified. Hence, even though Kant's deotology emphasizes duty and obligation, this does not mean infinite and unlimited toleration to an evil person.

It is easier to argue from a utilitarian perspective. In chapter five of Mill's Utilitarianism, he writes that the utilitarian ethical model is perfectly compatible with justice. If a person's wicked behavior undermines society's utility — and all other means cannot stop him from continuing —, then killing him would be justified since it is the choice that maximizes utility.

Now that we have justified the act of killing from two prominent normative ethical theories, we can now look at the question from a more practical perspective. China is the country that practices capital punishment in a magnitude that surpasses the sum of all other countries, yet China's criminal law is still very conservative about the use of capital punishment. Capital punishment is only considered if the criminal's conduct extremely endangers the security of society and if the highest court approves. Combining these two information together, one can see that capital punishment is indeed an unfortunate necessity.

Besides a legal angle, we can also look from the angle of war and diplomacy. During President Obama's presidency, Osama bin Laden was assassinated in a military assault. Is it justified? Killing the most wanted person who perpetrated the greatest act of terrorism in human history is indeed justified and brings forth a sense of justice.

Hence, from both a theoretical and practical angle, I believe that killing human beings can be justified.

It isn’t right to kill a human being but can be a necessity in very extreme situations.

• It is wrong to kill. I do not agree with the death penalty nor the principle of ‘an eye for an eye’. So, 99.9999999% of the time I would argue that it is never right to kill a human being. But there can be exceptions in extremely specific cases.
• For example, self-defence or killing to stop someone from taking a life are a couple of the explicit scenarios where I think it’s not as simple as obeying ‘do not kill’. I think this comes down to circumstance and should be considered on a case-by-case basis. Situations that come to mind are mass shootings or terrorist attacks where I think it can be reasonable to use excessive force to prevent further atrocities.
• I want to briefly explain the case of the Lindt Café siege in Sydney, Australia in December 2014, when an Islamic extremist held a café full of people hostage for 16 hours, eventually claiming the lives of two innocent people. I remember watching this unfold on the news and couldn’t take my eyes off what was happening. The attack culminated in police fatally shooting the attacker - a decision, I think, was necessary to prevent any further deaths of civilians.
• Therefore, I think it’s ok to believe that killing is wrong (as I do), but also recognise that sometimes it’s the only way of preventing further deaths.

It isn't NEVER right, Dignitas do a great job at ending suffering for people who desperately want that

The ethical stance on whether it is ever right to kill a human being is deeply complex and context-dependent, with strong arguments on both sides.

For:

Sanctity of Life: Many ethical and religious frameworks uphold the sanctity of human life, arguing that it is inherently valuable and should never be intentionally taken. This perspective emphasizes the moral duty to preserve life in all circumstances.

Human Rights: The right to life is a fundamental human right recognized by international law and human rights organizations. Deliberate killing violates this basic right and undermines the principles of justice and equality.

Moral Absolutism: Some ethical theories, such as deontology, argue that certain actions are intrinsically wrong, regardless of the consequences. Killing, from this viewpoint, is inherently immoral.

Against:

Self-Defense: Many ethical and legal systems recognize the right to kill in self-defense or to protect others from immediate harm. In these cases, killing may be seen as a necessary and justified action to prevent greater harm.

Just War Theory: Some ethical frameworks, like Just War Theory, argue that killing in the context of a just war can be morally permissible if it meets certain criteria, such as being a last resort and having a just cause.

Capital Punishment: While controversial, some argue that capital punishment is a justified response to certain heinous crimes, serving as a deterrent and a form of retributive justice. However, this view is highly debated and opposed by many human rights advocates.

Euthanasia: In cases of terminal illness and severe suffering, some ethical perspectives support euthanasia or assisted suicide as a compassionate choice, respecting the individual's autonomy and desire to avoid prolonged suffering.