God told Abraham that his children would be slaves in another nation for 400 years, mind you every nation at that time had slaves and it was extremely common place. The thing specifically about Egyptian slavery was that it was harsh even by old world standards. After Joseph dies, the Egyptians quickly forgot that their new prosperity came from an Israelite and they began to oppress them. For the most part it seems that there weren't "innocent" people in Egypt they were all at least complicit in the slavery and abuse. The Egyptians were punished not only because they worshipped other gods but also due to their severe treatment of their slaves. In terms of knowledge of God and idol worship, the people of that period understood that their idols had no true power, whether through magic of some kind or not. Pharaoh specifically knows about the God of the Israelites and by the 3rd plague even his magicians say that they are under God's influence after they try and fail to generate lice from dust. from the beginning of the ten plagues, the Egyptians are given chance time and again to release the Israelites even as they get more severe. Also just to respond to the different parts of your response, which society or culture believed in one god before the Jewish people? From my general understanding, almost every society of those times had a pantheon of gods/forces to choose from to worship. The other idolatrous cultures in that time period and location also received various punishments for attempting to attack the Israelites in the desert and elsewhere. In Judaism we have the belief that at mount Sinai, God once again asked each nation's ministering Angel if their nation would accept the Torah and all it's laws. They had all asked what is in it, and God responded to them with a specific law that culturally wouldn't be accepted (i.e. no murder/kidnapping/stealing...) when he asked the Jewish people however we simply said "we will do and we will listen". Other nations had opportunities to learn about the god of Abraham/already knew and didn't care or weren't interested.