The most famous Chinese assassins come from the Warring States period. Sima Qian's 《史记》 (Records of the Grand Historian) has a dedicated chapter called 《刺客列传》 (Biographies of Assassins), featuring Jing Ke (荆轲) who attempted to assassinate the King of Qin.
usage
In modern Chinese, 刺客 is common in gaming (assassin class), movies, and historical contexts. For a modern-day hired killer, 杀手 is more common.
