The correct answer to this open question is the following.
Some of the groups that began to fight slavery in the mid-1700s were Quakers and freed slaves.
These groups considered that slavery was inhumane and should be abolished. They thought that the slave trade was horrendous and that thousands of people died in the ships coming to America, from Africa. In a religious consideration, these groups considered that the practice of slavery was against God's will and should be ended in the Colonies.