Earlier in the film
VADER: "He will join us or die."
later
VADER: "Join me."
LUKE: "NEVER!"
[Vader's thoughts]: "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]: "Eat pit, Jedi."
The "Jedi Pit Theory" states that Jedi are nearly unassailable warriors except in the presence of pits. It points out how, in the OT, every time a Jedi is in trouble, they are near a pit (Ben dies on the Death Star, near a shaft; Palpatine dies when he falls into a pit, Luke is in trouble near the Sarlaac pit, etc)
