“Out of the crooked timber of humanity, no straight thing was ever made.”—Immanuel Kant.

For a more optimistic view:

“I hope there are none of you corrupted with the doctrine taught by wicked men for the worst purposes, and received by the malignant credulity of envy and ignorance, which is, that the men who act upon the public stage are all alike; all equally corrupt; all influenced by no other views than the sordid lure of salary and pension. The thing I know by experience to be false.
Never expecting to find perfection in men, and not looking for divine attributes in created beings, in my commerce with my contemporaries, I have found much human virtue. I have seen not a little public spirit; a real subordination of interest to duty; and a decent and regulated sensibility to honest fame and reputation. The age unquestionably produces (whether in a greater or less number than former times, I know not) daring profligates, and insidious hypocrites. What then? Am I not to avail myself of whatever good is to be found in the world, because of the mixture of evil that will always be in it? The smallness of the quantity in currency only heightens the value.
They who raise suspicions on the good on account of the behaviour of ill men, are of the party of the latter. The common cant is no justification for taking this party. I have been deceived, say they, by Titius and Maevius; I have been the dupe of this pretender or of that mountebank; and I can trust appearances no longer. But my credulity and want of discernment cannot, as I conceive, amount to a fair presumption against any man’s integrity.
A conscientious person would rather doubt his own judgment, than condemn his species. He would say, I have observed without attention, or judged upon erroneous maxims; I trusted to profession, when I ought to have attended to conduct. Such a man will grow wise, not malignant, by his acquaintance with the world. But he that accuses all mankind of corruption, ought to remember that he is sure to convict only one.
In truth I should much rather admit those, whom at any time I have disrelished the most, to be patterns of perfection, than seek a consolation to my own unworthiness, in a general communion of depravity with all about me.”—Edmund Burke

“Reality is that which, when you stop believing in it, doesn’t go away.” — Philip K. Dick.

“There is danger from all men. The only maxim of a free government ought to be to trust no man living with power to endanger the public liberty.”—John Adams

“Dissenters have been responsible for ending British control of the United States, for stopping slavery, for ending segregation, and for stopping an unwise war in Southeast Asia. In such situations, their opponents might well have said that the protesters hated America when really they despised flawed national policies.”— Margaret A. Blanchard

I respect kindness in human beings first of all, and kindness to animals. I don’t respect the law; I have a total irreverence for anything connected with society except that which makes the roads safer, the beer stronger, the food cheaper and the old men and old women warmer in the winter and happier in the summer.—Brendan Behan

“You can keep him up late at night, not roistering, but staring at a dead fire in a cold room. All the healthy and outgoing activities which we want him to avoid can be inhibited and nothing given in return, so that at last he may say, as one of my own patients said on his arrival down here, ‘I now see that I spent most of my life in doing neither what I ought nor what I liked.’ The Christians describe the Enemy as one ‘without whom Nothing is strong.’ And Nothing is very strong: strong enough to steal away a man’s best years not in sweet sins but in a dreary flickering of the mind over it knows not what and knows not why, in the gratification of curiosities so feeble that the man is only half aware of them, in drumming of fingers and kicking of heels, in whistling tunes that he does not like, or in the long, dim labyrinth of reveries that have not even lust or ambition to give them a relish, but which, once chance association has started them, the creature is too weak and fuddled to shake off.”