If the situation is truly that stark—misgendering one person is the only way to save one million lives—then many ethical frameworks would judge it morally justifiable.

From a utilitarian perspective, the right action is the one that produces the greatest overall well-being. The harm of misgendering someone (which can be significant and disrespectful) is far smaller than the harm of one million deaths. Preventing massive loss of life would outweigh the interpersonal harm.

From a rights- or dignity-based view, it’s more complicated because misgendering violates respect for a person’s identity. However, most versions of these theories still allow exceptions in extreme emergency cases where fundamental rights like the right to life are at stake for many others.

So the reasoning is: misgendering is normally wrong because it disrespects someone’s identity and dignity, but in an extreme, life‑or‑death scenario involving enormous numbers of people, the moral priority of preventing catastrophic harm can override that norm. The justification depends on the proportionality and the claim that there truly is no alternative way to save those lives.