Why did Lucius Malfoy look so sick in the last two Harry Potter movies?

Answer 1

Lucius Malfoy is manipulated by Lord Voldemort in the last book. At the beginning of Harry Potter and the Deathly Hallows, Voldemort takes Lucius' wand because his does not work against Harry's. Later, when the snatchers catch Harry, Ron and Hermione, Lucius' gets his son Draco to recognise Harry, so if they call Voldemort he will forgive them, but this does not happen because Dobby the House Elf rescues them and takes them to freedom.

During the final battle Lucius and his wife Narcissa change sides because they fear for Draco's safety. This results in them being spared Azkaban after the battle.

Answer 2

Lucius is under a huge amount of stress and pressure through Voldemort using Malfoy Manor as a sort of headquarters; he is living in a constant state of fear of what Voldemort might do to punish him and his family if they displease him (remember in Order of the Phoenix when Lucius dropped the prophecy - Voldemort was very unhappy about that); and he is also being physically abused by Voldemort as, in the book, Harry notices that Lucius has a puffy eye from a recent injury inflicted on him. Jason Isaacs (the actor who played as Lucius) described the character as being "broken" in the last two films.