I would tend to believe that he means that even though Vader holds a grudge towards him, killing him won't appease him and thus, by submitting to his lust of vengeance, he'd give power to Ben. I'm sure it is more complex and could be explained better, but this has allways been what I tought to be the underlying message. "If you strike me down, I shall be more powerful than you could ever imagine." In other words: I am ready to face my destiny (every being lives and DIE, no mather what...), but obviously, you aren't. It'll make me powerful beyond your imagination, as you seems to think you will not meet your fate at all, illusionned as you are by the darker side.