Rashi, citing a Midrash, asks a question. The Midrash answers with a proof-text. And he will redeem you from the house of slavery, from the hand of Pharaoh, the King of Egypt. From this we learn that Pharaoh enslaved all the Israelites for his own purposes. Therefore they were slaves to Pharaoh and only Pharaoh.
Slavery is a degrading and humiliating condition regardless of the master.
The earthly Kingdom resembles the heavenly one. The same systems which Hashem uses to operate the heavenly Kingdom are also operating in our world. This is the underlying idea of the blessing one makes upon seeing an earthly king. The honor that comes to a flesh and blood king is only because he somehow resembles the King of Kings. The Maharal of Prague explains that this is a positive state of affairs since the resemblance between the upper and lower Kingships points clearly to hand of Hashem at work.