Plagiarism is a penny ante thing, a measly squalor. It's like insider trading - a pervasive, petty, ploy that excites indignation and punishment, but, precisely because of its simmering ubiquity, fails to boil up to a real problem. Plenty of respectable people  pooh-pooh the tendency to get morally het up about it; they stand in front of stacks of stolen books and speeches and - like Frank Drebbin  in front of burning buildings - say Nothing to see here, nothing to see here...
Maybe they're right. It's easy to name high-profile plagiarists - Doris Kearns Goodwin , Charles Ogletree  - for whom it impeded their career not at all.
Yet for the most part universities and newspapers and publishing houses continue to respond with vehemence to its discovery; professors and journalists get fired, dissertations get revoked , novels get pulped. 
The novelist Jeremy Duns got burned by a friend's plagiarism. Q.R. Markham didn't plagiarize Duns, but Duns is angry at Markham for plagiarizing much of the content of his spy novel,  and pissed with himself for reading Markham's manuscript and failing to discover the plagiarism. Duns tries here  to figure out why he didn't discover it.
It's a thoughtful, scrupulous, account, but it's also funny, because it reveals a sort of infinite-regress plagiarism among spy novelists, in which the guy you plagiarized was himself plagiarizing, etc., etc.
Q. R. Markham  himself helps us understand the act. Markham writes Duns an honest-sounding (who knows?) account of his motives; and since his account jibes with UD's sense of what plagiarism is sometimes about - what the plagiarist-personality can be like - it's worth considering.
Many plagiarists are nowhere men, men without qualities, empty canvases, blank slates. All their lives they've elevated certain people into their superiors and worshiped them - people whose domineering personalities, intellectual brilliance, and Machiavellian cool puts their own meekness, mediocrity, and anxiety in the shade. Having grown up leeching off the energy, personal style, and ideas of their betters, these people make the act of writing an extension of their self-loathing, parasitic way of being.
Nothing astonishes and frightens the plagiarist more than the possibility that he or she has autonomy, a voice, independent skills. Thus Markham begins his letter to Duns by recounting his panic on realizing that because as a young poet he'd been selected for inclusion in a Best of collection, this has to have meant that he was indeed the best, and in order to keep being the best he'd have to steal from people who actually were the best. Or a whole lot better.
Note that it never occurs to Markham to put what has happened to him into any sort of perspective; note that he takes the word Best in the collection literally, at face value, strictly as given. Since he knows that, at nineteen, he can't be the best, one would have expected him to work out what that amazing bit of early acclaim actually meant. Perhaps it meant that he was very promising, and that he should take his having been selected as encouragement toward greater artistic maturity, etc. But instead:
I decided I had to be good then and there. Because I was already supposed to be the Best... I began to distrust my own voice and began swiping other people's words or phrases because I thought they sounded better or more clever than my own. Perhaps if there had been no pressure to keep publishing it might have been different, but in my mind my course was set.
His course was set because someone had suddenly set it. The plagiarist has little in the way of personal enterprise; he waits, as Henry James wrote about women in The Portrait of a Lady, for the world to to furnish him with a destiny. And because he has always instinctively looked to his superiors for his entire way of life and thought, he now - having had writing thrust upon him - looks to his literary betters.
Somehow public scrutiny has always been the pressure point for me. Once I feel I'm doing the work for someone else's eyes, I begin stealing, because I want to impress.
A passive nonentity he may be, but if circumstances conspire to lift the plagiarist to public scrutiny, he'll revel in it. His relationship with his heroes - he wants to impress them with how good a job he does copying their ways - now becomes his relationship to his writing and to the world. What Markham calls his "people pleasing" - his need to be seen as the commanding writer people think he is - provokes "a strange schizophrenic form of gambling" in him: He knows on some level that he'll be found out, but he panics and bets on that not happening.
Not sure [how to describe my motives]: the fantasy that I had written these words? That I, too, could be a world-class spy novelist? Whatever it was, I know there was some kind of built-in death wish, for as you say: these are not obscure authors.
That death wish is directed not merely at himself. So far the plagiarist of this sort looks like an almost-innocent object of pity - a lame, immature, narcissistic loser who can't help it and who only hurts himself. But in these plagiarists there may also be a good deal of resentment-driven aggression against the world. Their various life failures have generated a cynical philosophy in which everything out there is just a nasty game, and they have as much right to play it, dammit, as anyone else. Life is an exciting, amoral gamble.
I can relate to the figure of the cold and amoral spy who is sent off on a mission to do bad things for his government and has to dissemble...
At its worst, the plagiarist's resentment about his own meagerness may generate, along with hero-worship of his betters, serious hostility toward them. What better way to bump them off than stealing their souls? Being them?
Plagiarists are the Mark David Chapmans of the writing world.