A liberator is someone who sets people free from captivity. Abolitionists were liberators who fought to free African-American slaves from bondage in the years before the Civil War.
Both liberator and liberty derive from the Latin liberare meaning "to set free." A liberator is someone who provides liberty, or freedom, to people held captive or repressed. At the end of the Holocaust, allied forces entered Germany and Poland, acting as liberators for millions of Jews held in concentration camps during World War II.