When?

When the Romans ruled the known world, they had, among others, a simple weapon of terror they used to keep the soldiers in their army literally in line during battle.

Decimation: a form of extreme military discipline used by officers in the Roman Army to punish mutinous or cowardly soldiers. The word decimation is derived from Latin meaning "removal of a tenth." A cohort selected for punishment by decimation was divided into groups of ten; each group cast lots, and the soldier on whom the lot fell was executed by his nine comrades, often by stoning or clubbing. The remaining soldiers were given rations of barley instead of wheat and forced to sleep outside of the Roman encampment.
Because the punishment fell by lot, all soldiers in the selected cohort were eligible for execution, regardless of rank or distinction. As a result, the threat of decimation inspired fear and resolve into the Roman Legions. However, because a decimation significantly reduced the troop strength of an army, it is believed that the punishment was rarely used. (Wikipedia)

When an agent of an occupying army or government is attacked and killed by a resistance movement, this has, upon occasion, led to civilian reprisals.

“In warfare, a reprisal is a limited and deliberate violation of the laws of war to punish an enemy for breaking the laws of war. A legally executed reprisal is not an atrocity.
To be legally justified, a reprisal can only be directed against the party carrying out the original violation, can only be carried out as a last resort, after having given formal notice of the planned reprisal, must be proportionate to the original violation, must have the aim of persuading the original violator to comply with the legally accepted behaviour in future, and must not continue after the illegal behaviour ends.
Circumstances usually dictate that reprisals can only be taken against people innocent of the original violation, which increases the probability that the reprisals will themselves be viewed as hostile acts, risking a vicious circle of violations by both sides.
All four Geneva Conventions prohibit reprisals against, respectively, battlefield casualties, shipwreck survivors, prisoners of war and civilians, as well as certain buildings and property. An additional 1977 protocol extends this to cover historic monuments, works of art and places of worship. Despite this, in ratifying the Conventions, a number of states have reserved the right to undertake reprisals against civilians, including the United Kingdom and Germany.
In the United States military, the lowest ranked commander who can authorize a reprisal is a general in command of a theater.”

Many times throughout history, civilian reprisals have been carried out on the thin justification that the victims of the reprisal were suspected of aiding and abetting the perpetrators of a violent act. In fact, no effort was made to justify these suspicions because the reprisal itself was staged as a warning to the perpetrators to the effect that their continued actions would result in even more physical harm to their friends and neighbors. It was also an attempt to strike fear into the civilian population concerning what would happen to them if they were even suspected of rendering aid to the rebels. In short, this is another weapon of terror and an attempted extortion of good behavior.

When a government or other authority condones a massacre or genocide either through direct action or deliberate failure to take preventive action, that authority is said to have committed a crime against humanity.

“A crime against humanity is a term in international law that refers to acts of persecution or any large scale atrocities against a body of people, as being the criminal offence above all others.[1]
The Rome Statute Explanatory Memorandum states that crimes against humanity "are particularly odious offences in that they constitute a serious attack on human dignity or grave humiliation or a degradation of one or more human beings. They are not isolated or sporadic events, but are part either of a government policy (although the perpetrators need not identify themselves with this policy) or of a wide practice of atrocities tolerated or condoned by a government or a de facto authority. However, murder, extermination, torture, rape, political, racial, or religious persecution and other inhumane acts reach the threshold of crimes against humanity only if they are part of a widespread or systematic practice. Isolated inhumane acts of this nature may constitute grave infringements of human rights, or depending on the circumstances, war crimes, but may fall short of meriting the stigma attaching to the category of crimes under discussion. "[2]

When the alleged Almighty God Jehovah decided that his allegedly chosen people had been slaves in Egypt long enough, he caused the death of the Egyptian’s firstborn children. There is no recorded age limit. Presumably, a young Egyptian couple with their first newborn suffered the same fate and lost their only child. These children were killed because their Pharaoh, their parent’s ruler and government, would not condone the release of the Israeli slaves. Setting aside the suspicious nature of the ten plagues (more on this later), and the possible agencies behind these “miracles”, if you believe in the inerrancy of the Bible, then this took place.

When is this ever right? I’m not looking for excuses or moral justifications, I want to know when is it right to murder innocent human beings in order to achieve social engineering?