I think DVK's answer is great, but its also helpful to look at Vader's character more deeply, specifically that "He is machine now more than man." Machines are just motivated by their programming logic.
Vader was originally motivated by his desire to save Padme. When Padme died, his motivation was lost, and like his body, was replaced with a robotic state of obedience to the Dark Side. The only thing he knew to do after that was to obey his master.
But Luke brought out the "good in him", restoring his humanity. Even in their first encounter, Vader briefly moved out of his robotic obedience by offering to overthrow the Emperor and rule as father and son. But Luke rejected his offer and in their next encounter the robot returned, "I must obey my master".
In the end, Vader found a human motivation to live for, the motivation to save his son. It overrode his programming and restored his humanity.