“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 —
(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?