1) Crusader 2) Emancipationist 3) Meliorist 4) Reformer 5) Reformist 
• (n.) A person who favors the abolition of any institution, especially negro slavery.
Ab`o·li'tion·ist noun A person who favors the abolition of any institution, especially negro slavery. 
[n] - a reformer who favors abolishing slavery
abolitionist 1. Someone who supports the abolition of something; such as, a system, a practice, or an institution. 2. A reformer who favors abolishing slavery.
emancipationist noun a reformer who favors abolishing slavery
one who favors doing away with slavery.
(esp. prior to the Civil War) a person who advocated or supported the abolition of slavery in the U.S. · a person who favors the abolition of any law or practice deemed harmful to society: the abolitionists who are opposed to capital punishment.
a reformer who favors putting an end to slavery