The word "slavery" has a negative connotation, largely due to the exploitation (or abuse) associated with it in the past few centuries. However, if slavery is chosen by a person on one's own volition leading to one's own benefit (or for larger good), then it is acceptable and it is sometimes praiseworthy. We indulge in such "slavery" often in life as in the case of an individual bidding the orders of superiors even if it means going against once own conscience: it is seen in armed forces, any organization, a family or in practicing religion also.
In religion, one always comes to this cross-roads in one's journey (usually more than once): "should I follow whatever is said by a person (or written in a scripture) blindly like a 'slave' or should I follow one’s conscience (could be in-line with or against scriptures) and learn from the consequences of my own actions like a self-driven individual?" On the face of it, one may say the former path is simpler and easier to follow as there is no need to re-invent the wheel as seen in following the latter path - assuming the personage and scripture is the right one to be followed (not an easy one to determine). On the contrary, I would say both the paths are necessary and it is up to the individual to apply the appropriate path in each case. More often than not, it is the latter path of following one’s conscience that helps a person to learn a great deal in life. Usually, people glibly assent to a particular doctrine mentioned by a great personage or scripture, but that assent has no significant impact on transforming their lives. It would be better in such a case to actually go against the doctrine, if necessary, and face the consequences of that action as one's life would then undergo a real transformation for the better.
Broadly speaking, the attitude of "slavery" to a personage/scripture forms the cornerstone in the path of devotion whereas in the path of analysis the usual method is to apply self-effort based on one's conscience and learn through its consequences: recall the parable of kitten (resigned to its mother) and baby-monkey (clings to the mother by its own will) told by Sri Ramakrishna in this context. Many of us are of little faith and it is better for us to go through the path of self-analysis than delude ourselves to think that we are devoted to an ideal and then do nothing. Lord Jesus saying is apt here: "Because you have so little faith. Truly I tell you, if you have faith as small as a mustard seed, you can say to this mountain, 'Move from here to there,' and it will move. Nothing will be impossible for you." (Matthew 17:20). But this ultimate faith or resignation to Higher Self comes only after the individual has completely exhausted all the options of self-analysis/self-effort. The parable told by Sri Ramakrishna: 'a bird on the mast of the ship that sailed away from the port leading to it frantically flying in all directions, only to find no land nearby and then resigning itself calmly sits on the mast again' is a good illustration.