"Yes, Voldemort is playing a very clever game. Declaring himself might have provoked open rebellion: remaining masked has created confusion, uncertainty, and fear." (11.35)
Lupin notes that Voldemort really knows how best to use his power – the most alarming thing is that he keeps things mysterious, so that nobody knows exactly how strong he is or what he's capable of.
The Patronus, he was sure, was Umbridge's and it glowed brightly because she was so happy here, in her element, upholding the twisted laws she had helped to write. (13.70)
Power warps people – and Dolores Umbridge is a prime case study for this phenomenon. Her love for power is unnatural, and it's rendered her an unnatural character; she delights in exhibitions of her strength, and prefers them to be at the expense of others.
"Yes, we have been given power and yes, that power gives us the right to rule, but it also gives us responsibilities over the ruled. We must stress this point, it will be the foundation stone upon which we build… where we meet resistance, we must use only the force that is necessary and no more." (18.34)
This quote, from a letter that young Dumbledore wrote to Grindelwald, demonstrates the dangers of having too much power – the idea that the added strength of magic makes wizards fit to rule (albeit "responsibly") over Muggles is the overly enthusiastic and dangerously idealistic claim of an immature, young wizard.
"… the wand would be bound to attract trouble –."
"Only if you shouted about it," argued Ron. "Only if you were prat enough to go dancing around, waving it over your head, and singing, 'I've got an unbeatable wand, come and have a go if you think you're hard enough.' As long as you could keep your trap shut –."
"Yes, but could you keep your trap shut?" said Hermione, looking skeptical. (21.57)
Hermione brings up a good point here – one of the greatest dangers of having a ton of power is the temptation to brag about it and thus get yourself in more trouble than said power is worth.
If only there was a way of getting a better wand…
And desire for the Elder Wand, the Deathstick, unbeatable, invincible, swallowed him once more… (22.55-56)
Even Harry wonders for a moment if more power might be the answer – the temptation of the Elder Wand is a danger to any wizard in a fix, even our hero.
"The Dark Lord no longer seeks the Elder Wand only for your destruction, Mr. Potter. He is determined to possess it because he believes it will make him truly invulnerable."
"And will it?"
"The owner of the Elder Wand must always fear attack," said Ollivander, "but the idea of the Dark Lord in possession of the Deathstick is, I must admit… formidable."
Harry was suddenly reminded of how he had been unsure, when they first met, of how much he liked Ollivander. Even now, having been tortured and imprisoned by Voldemort, the idea of the Dark wizard in possession of this wand seemed to enthrall as much as it repulsed him. (24. 156-157)
Mr. Ollivander's moral ambiguity is caused by his fascination with the idea of so much power existing in one place. On a certain level, he's more interested in the concept of the most powerful living wizard possessing the most powerful wand, even if it's used for evil, in a kind of scientific way.
"That which Voldemort does not value, he takes no trouble to comprehend. Of house-elves and children's tales, of love, loyalty, and innocence, Voldemort knows and understands nothing. Nothing. That they all have a power beyond his own, a power beyond the reach of any magic is a truth he has never grasped." (35.28)
Ah… so, real power, it seems, springs from love, something Dumbledore has said all along, but that Voldemort has never listened to. And this is why Harry triumphs over the Dark Lord in the end; he recognizes, unlike his foe, that he isn't alone in the world and wouldn't want to be, and that it's love that makes the world go 'round.
"Maybe a man in a million could unite the Hallows, Harry. I was fit only to possess the meanest of them, the least extraordinary. I was fit to own the Elder Wand, and not to boast of it, and not to kill with it. I was permitted to tame and to use it, because I took it, not for gain, but to save others from it.
"But the Cloak, I took out of vain curiosity, and so it could never have worked for me as it works for you, its true owner. The stone I would have used in an attempt to drag back those who are at peace, rather than to enable my self-sacrifice, as you did. You are the worthy possessor of the Hallows." (35.84-85)
Dumbledore recognizes wisely that power works best for those who deserve it – he could never have controlled all three Hallows because he never merited them. This is something that Voldemort, who thinks he can seize whatever power he wants, will never understand.
"I'm putting the Elder Wand," he told Dumbledore, who was watching him with enormous affection and admiration, "back where it came from. It can stay there. If I die a natural death like Ignotus, its power will be broken, won't it? The previous master will never have to be defeated. That'll be the end of it.
Dumbledore nodded. They smiled at each other.
"Are you sure?" said Ron. There was the faintest trace of longing in his voice as he looked at the Elder Wand.
"I think Harry's right," said Hermione quietly.
"That wand's more trouble than it's worth," said Harry. "And quite honestly… I've had enough trouble for a lifetime." (36.147-150)
Harry shows true power of a sort here – the ability to say "no" to having too much power, if that makes sense. His decision to break the pattern of the Elder Wand and to lay it to rest again in Dumbledore's tomb proves his worthiness in ever possessing the Hallows; he knew when to use them, and when to stop.