I have sometimes been ready to think that the passion for Liberty cannot be Eaquelly Strong in the Breasts of those who have been accustomed to deprive their fellow Creatures of theirs.
Abigail is pointing out a contradiction in the revolutionary mindset. This shows that thoughtful colonists were fully aware of the hypocrisy of fighting a war for "liberty" from Britain while keeping enslaved people and denying basic legal rights to married women at home.