ʿAbdullāh b. ‘Amr – Allāh be pleased with them both – said:

Killing a believer is worse to Allāh than the ending of the world.

Al-Nasā'ī, ­Al-Sunan. Graded sahīh by Shaykh Al-Albānī in Sahīh Al-Jāmi’ no.4361.