IPA: /əˈsæsɪneɪtər/
KK: /ə'sæsɪneɪtər/
A person who kills someone secretly and often for political reasons.
The assassinator was hired to eliminate the target without being detected.
Assassinator is formed from "assassin" (from Old French *assassiner*, meaning to kill) and the suffix "-ator" (meaning one who performs an action). The word "assassinator" refers to one who kills or carries out an assassination.
Think of someone who 'kills' ('assassin') and the '-ator' means a person who does that action. An assassinator is someone who performs the act of killing.
No commonly confused words.