´It was narrated from 'Abdullah bin 'Amr that:` The Prophet [SAW] said: "The extinction of the whole world is less significant before Allah than killing a Muslim man."
Related hadith on this topic
Explanation & Benefits
Hafiz Muhammad Ameen
That is, if the world is hypothetically assumed to be without believers, then the destruction of the world and all that is in it is lighter in the sight of Allah Ta'ala than the unjust loss of the life of a single believer. Or, if, hypothetically, the entire world—which is assumed to be devoid of believers—is destroyed, then its sin is much less than the sin of the unjust killing of a single believer. The purpose is to demonstrate the importance of the believer and of faith, which has been illustrated through this hypothetical scenario. This style is common in customary usage.
Source: Sunan Nasa'i: Translation and Benefits by Shaykh Hafiz Muhammad Amin Hafizullah, Page: 3991