That’s true. Paradise Lost tells us about the fall of Adam and Eve, and their sin. Thus, undoubtedly the most prominent theme of this poem is sin. Remember that we also learn about major sin of Satan, who attempts to remove God from power and it tells us about the sins of Adam’s descendants, who would commit sins in future.
Oh, yes it also a revenge tragedy. Satan tries to seek revenge upon God by using innocent couple. He feels bad to see them together, however, he continues with his demonic plan. In the long run, the winner will be Adam and Eve, nevertheless in this poem nobody wins.