"It's a happy ending, until you realize that Parasitis consumed your home planet prior to the events of the game, and now it's pretty much just you and the princess, adrift in space. Was it all worth it...?"
I think you answered your own question with the previous sentence.
oh how I remember as a kid longing for a gory game.... at the time all I had was Monster Party which got nasty half way through stage 1, but this, this little gem was one i wanted for a while... and Splatterhouse ( which I now have )
I remember reading that the metallic sections of the parasite are the remains of a medical ship Parasitis swallowed. Incidentally, that's also how a princess wound up in there in the first place; she was a passenger on that ship.