What does 'be done' mean?

Question

“Seize him! SEIZE HIM!” shrieked Voldemort again, and Quirrell lunged, knocking Harry clean off his feet landing on top of him, both hands around Harry’s neck — Harry’s scar was almost blinding him with pain, yet he could see Quirrell howling in agony.
“Master, I cannot hold him — my hands — my hands!”
And Quirrell, though pinning Harry to the ground with his knees, let go of his neck and stared, bewildered, at his own palms — Harry could see they looked burned, raw, red, and shiny.
“Then kill him, fool, and be done!” screeched Voldemort.
Quirrell raised his hand to perform a deadly curse, but Harry, by instinct, reached up and grabbed Quirrell’s face —
“AAAARGH!”
(Harry Potter and the Sorcerer's Stone)

‘Be done’ seems to mean ‘Kill the boy’. But I can’t understand the structure of ‘be + adjective’, which seems not take object. What does the phrase mean, and how do I understand the structure?

Answer 1

“Be” is an imperative. You've probably encountered that construction with other adjectives such as “be quiet” (act in such a manner as not to make noise), “be still” (act in such a manner as not to move), “be on time” (act in such a manner as not to be late), …

The adjective “done” means finished with a particular action. “Be done” is an idiom. If you take it literally, it means either “arrange to have finished now”, which cannot be done, or “arrange to have finished in the future”, which would mean “do it”. What “be done” actually means is “finish this soon”: it's a command to complete a task quickly.

In this passage, Voldemort is unhappy that Quirrell is taking too much time killing Harry, and instructing him to finish the task so that they can move on to do other things.