massacre
殲 centers on total destruction of life: a massacre, wiping out completely.
殲 combines 歹 (death) with 韱, which likely contributes the sound and may relate to cutting or destroying. The exact historical development of 韱 is uncertain.
The death radical 歹 beside the complex 韱 suggests a thorough, violent end. Imagine a scene of total destruction where nothing is left alive.
For セン, picture a thousand (千, せん) lives being wiped out in a massacre: 千 -> セン, and the death radical 歹 marks the grim scene.