Majestic is an allusion to the idea that the angel's, even from Hell, still think of themselves as heavenly creatures, even after they have fallen. The word majestic brings thoughts of glory and heaven, and the delusion is that even from hell, even after being punished by god, these fallen one's and most specifically Beelzebub still see themselves as the heavenly creation that have not fallen but rather are on a hiatus from heaven and with or without God they will one day return. But while we can take majestic in a positive connotation, according to the Oxford English Dictionary, it means nothing more than something "possessing or characterized by majesty; of imposing dignity or grandeur". This would seem to give the reader a feeling of emptiness in the characterization of Beelzebub. It would seem that all this majesty and grandeur is actually nothing more than vanity, now that the angel's have fallen to Hell.
Through use of the word "ruin", Milton reminds us that although these angel's look princely and majestic and royal, they are indeed still flawed and fallen. The word ruin reminds the reader that these angels now know sin, and that this majestic look is nothing more empty vanity.
-Erik Scott