This is my favorite passage from the book (added: major spoilers for the ending):

"Indeed. Before becoming a truly terrible Dark Lord for David Monroe to fight, I first created for practice the persona of a Dark Lord with glowing red eyes, pointlessly cruel to his underlings, pursuing a political agenda of naked personal ambition combined with blood purism as argued by drunks in Knockturn Alley. My first underlings were hired in a tavern, given cloaks and skull masks, and told to introduce themselves as Death Eaters."

The sick sense of understanding deepened, in the pit of Harry's stomach. "And you called yourself Voldemort."

"Just so, General Chaos." Professor Quirrell was grinning, from where he stood by the cauldron. "I wanted it to be an anagram of my name, but that would only have worked if I'd conveniently been given the middle name of 'Marvolo', and then it would have been a stretch. Our actual middle name is Morfin, if you're curious. But I digress. I thought Voldemort's career would last only a few months, a year at the longest, before the Aurors brought down his underlings and the disposable Dark Lord vanished. As you perceive, I had vastly overestimated my competition. And I could not quite bring myself to torture my underlings when they brought me bad news, no matter what Dark Lords did in plays. I could not quite manage to argue the tenets of blood purism as incoherently as if I were a drunk in Knockturn Alley. I was not trying to be clever when I sent my underlings on their missions, but neither did I give them entirely pointless orders -" Professor Quirrell gave a rueful grin that, in another context, might have been called charming. "One month after that, Bellatrix Black prostrated herself before me, and after three months Lucius Malfoy was negotiating with me over glasses of expensive Firewhiskey. I sighed, gave up all hope for wizardkind, and began as David Monroe to oppose this fearsome Lord Voldemort."

"And then what happened -"

A snarl contorted Professor Quirrell's face. "The absolute inadequacy of every single institution in the civilization of magical Britain is what happened! You cannot comprehend it, boy! I cannot comprehend it! It has to be seen and even then it cannot be believed! You will have observed, perhaps, that of your fellow students who speak of their family's occupations, three in four seem to mention jobs in some part or another of the Ministry. You will wonder how a country can manage to employ three of its four citizens in bureaucracy. The answer is that if they did not all prevent each other from doing their jobs, none of them would have any work left to do! The Aurors were competent as individual fighters, they did fight Dark Wizards and only the best survived to train new recruits, but their leadership was in absolute disarray. The Ministry was so busy routing papers that the country had no effective opposition to Voldemort's attacks except myself, Dumbledore, and a handful of untrained irregulars. A shiftless, incompetent, cowardly layabout, Mundungus Fletcher, was considered a key asset in the Order of the Phoenix - because, being otherwise unemployed, he did not need to juggle another job! I tried weakening Voldemort's attacks, to see if it was possible for him to lose; at once the Ministry committed fewer Aurors to oppose me! I had read Mao's Little Red Book, I had trained my Death Eaters in guerilla tactics - for nothing! For nothing! I was attacking all of magical Britain and in every engagement my forces outnumbered their opposition! In desperation, I ordered my Death Eaters to systematically assassinate every single incompetent managing the Department of Magical Law Enforcement. One paper-pusher after another volunteered to accept higher positions despite the fate of their predecessors, gleefully rubbing their hands at the prospect of promotion. Every one of them thought they would cut a deal with Lord Voldemort on the side. It took seven months to murder our way through them all, and not a single Death Eater asked why we were bothering. And then, even with Bartemius Crouch risen to Director and Amelia Bones as Head Auror, it was still too little. I could have done better fighting alone. Dumbledore's aid was not worth his moral restraints, and Crouch's aid was not worth his respect for the law." Professor Quirrell turned up the fire beneath the potion.

"And eventually," Harry said through the heart-sickness, "you realized you were just having more fun as Voldemort."

"It is the least annoying role I have ever played. If Lord Voldemort says that something is to be done, people obey him and do not argue. I did not have to suppress my impulse to Cruciate people being idiots; for once it was all part of the role. If someone was making the game less pleasant for me, I just said Avadakedavra regardless of whether that was strategically wise, and they never bothered me again." Professor Quirrell casually chopped a small worm into bits. "But my true epiphany came on a certain day when David Monroe was trying to get an entry permit for an Asian instructor in combat tactics, and a Ministry clerk denied it, smiling smugly. I asked the Ministry clerk if he understood that this measure was meant to save his life and the Ministry clerk only smiled more. Then in fury I threw aside masks and caution, I used my Legilimency, I dipped my fingers into the cesspit of his stupidity and tore out the truth from his mind. I did not understand and I wanted to understand. With my command of Legilimency I forced his tiny clerk-brain to live out alternatives, seeing what his clerk-brain would think of Lucius Malfoy, or Lord Voldemort, or Dumbledore standing in my place." Professor Quirrell's hands had slowed, as he delicately peeled bits and small strips from a chunk of candle-wax. "What I finally realized that day is complicated, boy, which is why I did not understand it earlier in life. To you I shall try to describe it anyway. Today I know that Dumbledore does not stand at the top of the world, for all that he is the Supreme Mugwump of the International Confederation. People speak ill of Dumbledore openly, they criticize him proudly and to his face, in a way they would not dare stand up to Lucius Malfoy. You have acted disrespectfully toward Dumbledore, boy, do you know why you did so?"

HPMOR (discussion & meta)