Little Birds

Hunger perhaps ?

Shock value. The same goes for Tommen jumping. Don't bother looking for an explanation, the writers wanted a shock scene. The Little Birds are spies, not killers and having untrained children kill a man for the first time in group is something no one would ever try to do.

Having said that, though, the book ends pretty much the same way, except that they finish Kevan Lannister with daggers after Varys has shot him with a crossbow. GRRM is not better than D&D when it comes to indulging in that sort of implausible horrors.

Then he saw. Grand Maester Pycelle was seated at his table, his head pillowed on the great leather-bound tome before him. Sleeping, Kevan thought … until he blinked and saw the deep red gash in the old man’s spotted skull and the blood pooled beneath his head, staining the pages of his book. All around his candle were bits of bone and brain, islands in a lake of melted wax.
He wanted guards, Ser Kevan thought. I should have sent him guards.
ould Cersei have been right all along? Was this his nephew’s work? “Tyrion?” he called. “Where …?”
“Far away,” a half-familiar voice replied.
He stood in a pool of shadow by a bookcase, plump, pale-faced, round-shouldered, clutching a crossbow in soft powdered hands. Silk slippers swaddled his feet.
“Varys?”
The eunuch set the crossbow down. “Ser Kevan. Forgive me if you can. I bear you no ill will. This was not done from malice. It was for the realm. For the children.”
I have children. I have a wife. Oh, Dorna. Pain washed over him. He closed his eyes, opened them again. “There are … there are hundreds of Lannister guardsmen in this castle.”
“But none in this room, thankfully. This pains me, my lord. You do not deserve to die alone on such a cold dark night. There are many like you, good men in service to bad causes … but you were threatening to undo all the queen’s good work, to reconcile Highgarden and Casterly Rock, bind the Faith to your little king, unite the Seven Kingdoms under Tommen’s rule. So …”
“Are you cold, my lord?” asked Varys. “Do forgive me. The Grand Maester befouled himself in dying, and the stink was so abominable that I thought I might choke.”
Ser Kevan tried to rise, but the strength had left him. He could not feel his legs.
“I thought the crossbow fitting. You shared so much with Lord Tywin, why not that? Your niece will think the Tyrells had you murdered, mayhaps with the connivance of the Imp. The Tyrells will suspect her. Someone somewhere will find a way to blame the Dornishmen. Doubt, division, and mistrust will eat the very ground beneath your boy king, whilst Aegon raises his banner above Storm’s End and the lords of the realm gather round him.”
“Aegon?” For a moment he did not understand. Then he remembered. A babe swaddled in a crimson cloak, the cloth stained with his blood and brains. “Dead. He’s dead.”
“No.” The eunuch’s voice seemed deeper. “He is here. Aegon has been shaped for rule since before he could walk. He has been trained in arms, as befits a knight to be, but that was not the end of his education. He reads and writes, he speaks several tongues, he has studied history and law and poetry. A septa has instructed him in the mysteries of the Faith since he was old enough to understand them. He has lived with fisherfolk, worked with his hands, swum in rivers and mended nets and learned to wash his own clothes at need. He can fish and cook and bind up a wound, he knows what it is like to be hungry, to be hunted, to be afraid. Tommen has been taught that kingship is his right. Aegon knows that kingship is his duty, that a king must put his people first, and live and rule for them.”
Kevan Lannister tried to cry out … to his guards, his wife, his brother … but the words would not come. Blood dribbled from his mouth. He shuddered violently.
“I am sorry.” Varys wrung his hands. “You are suffering, I know, yet here I stand going on like some silly old woman. Time to make an end to it.” The eunuch pursed his lips and gave a little whistle.
Ser Kevan was cold as ice, and every labored breath sent a fresh stab of pain through him. He glimpsed movement, heard the soft scuffling sound of slippered feet on stone. A child emerged from a pool of darkness, a pale boy in a ragged robe, no more than nine or ten. Another rose up behind the Grand Maester’s chair. The girl who had opened the door for him was there as well. They were all around him, half a dozen of them, white-faced children with dark eyes, boys and girls together.
And in their hands, the daggers.