Yes, that one line was really cryptic and mysterious. I too wondered what it was really supposed to mean when re-watching the film time to time. To me, it was either supposed to convey that Vader was so corrupted by the Dark Side and its powers that he couldn't go back, or as you mention, that the Emperor was so powerful and would retaliate in such a way that he couldn't bring himself to turn against him....Who knows. And I kept wondering for a long time what Lucas and/or Kasdan had in mind when they came up with that particular line. So far, it's still a mystery to me.