From Lexicanum

M'Shen was the Callidus assassin who killed the rebel Primarch Night Haunter. Evidentially he already knew he was the target of an assassin, and only allowed himself to be killed to "vindicate" his rebellion against the Imperium.

M'shen is likely a reference to the real life actor Martin Sheen, who assassinates rebel Colonel Walter E. Kurtz in the film Apocalypse Now (who himself is inspiration for Primarch Konrad Curze).

Sources

• Index Astartes II