*Earlier in the film*<br />
"He will join us or die." - Vader

*later*<br />
"Join me." - Vader<br />
"NEVER!" - Luke<br />
"So...let my son be tortured to the breaking point by Sidious, or kill him...oh look, he's falling.  Decision made." - Vader's thoughts<br />
"[Eat pit, Jedi.][1]" - Vader


  [1]: http://www.galacticsenate.com/showthread.php?10294-The-Jedi-Pit-Theory