This is definitely out-of-character for Lucius Malfoy.
He has on numerous occasions shown to be:
- Definitely not outwardly courageous or rash
He is also an extremely (if not the most) important Death Eater still alive / at large. So he would not go so far as to expose himself with such a controversial act.
That being said, there is definitely no mention or evidence of "Avada..." leading to anything other than "...Kedavra"; so it is fair to assume that he was going to use the Killing Curse in the movie.
It is important to mention here as well that (as the OP pointed out) this is only in the film version; in the books he merely "lunges" at Harry:
Lucius Malfoy stood frozen, staring at the elf. Then he lunged at Harry.
“You’ve lost me my servant, boy!”
But Dobby shouted, “You shall not harm Harry Potter!”
There was a loud bang, and Mr. Malfoy was thrown backward.
Harry Potter and the Chamber of Secrets, Chapter Eighteen, "Dobby's Reward"
So why the discrepancy?
Personally I feel that it was the writers' and the actor's way of adding effect to Lucius wielding his wand and dramatising the whole affair. He is extremely upset at both his diary ploy being foiled as well as losing his family's servant.
Lucius has a cunning attitude as well as a strong influence both in the Ministry and with the Hogwarts Governance Board; he likely could have tried to murder Harry and then:
- Pass the blame onto his servant, Dobby
- Accuse Harry of attacking him first and claim Self-Defence
Both are viable and fit in with his character, as well as his high influence and control over members of the Ministry or the Wizengamot.