Most appropriate word for proxy/substitute/surrogate
proxy: 'the authority to represent someone else, especially in voting' is "代理" or "代理人" in Chinese

surrogate: 'a substitute, especially a person deputizing for another in a specific role or office' is also "代理" or "代理人" in Chinese

The baseball term "代打" (substitute batter) also obtained the meaning of 'substitute' (for any role)


  For the sake of clarity, let's say you wanted to kill someone, but wanted someone else to do it for you.


借刀殺人 is a common idiom for "arrange someone to kill for you". It literally means "borrow knife to kill" and the "borrowed knife" is someone other than yourself. He can be your willing agent, or an unwitting pawn

假手於人 is a common idiom for "make someone else to do your job"