Harry Potter and the Deathly Hallows
How we cite our quotes:
"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.