Disgraced Maester Qyburn has been a growing figure in the world of Game of Thrones since season three, and on Sunday night's season six finale, he established himself as a major player by helping Cersei orchestrate a plot to destroy her enemies and seize the Iron Throne — and in the process he handily destroyed his rival, Grand Maester Pycelle.
Qyburn has been one of Cersei's most crucial allies, especially so since her imprisonment by the Sparrows, and the end of season six saw him standing, quite literally, at her right hand.
Part of his ascent to power was helping to mastermind the plan to use wildfire to destroy the Great Sept of Baelor and everyone in it — but Qyburn took special care with Pycelle, dispatching his "little birds" to stabby-stab him to death. And there's a reason why Qyburn took extra measures to kill Pycelle.
Maester Pycelle had warned Cersei about the "arrogance that had [Qyburn] expelled from the Citadel," which clearly won him no favors with Qyburn.
It would likely have been easy for Cersei and Qyburn to make sure that Pycelle was in the Sept along with everyone else, which would have meant he'd have been killed in the explosion. But instead, perhaps as payback for Pycelle's warnings about Qyburn, Qyburn took pains to lure him into a secluded hallway and then urged his "little birds," aka his army of creepy children, to gruesomely murder the old Grand Maester.
This death does several things for Qyburn — first, it shows the extent of the loyalty of his "little birds," whom he won over from Varys with gifts of candy; those kids are willing to kill for Qyburn. Second, killing Pycelle establishes Qyburn as the new maester in town (even though he technically lost his license or whatever it is that qualifies maesters).
Pycelle may have been a terrible, flawed person, but by killing him the way he did, Qyburn sent a message that a new, darker era has arrived at King's Landing. But we'll have to wait for season seven to see how far Qyburn will get with his plans before he's interrupted by the arrival of the fleet of enemies headed for the Red Keep.