I have long held the contention that, much as we generally claim to value purity in sport, we also long to see an unnatural performance. This is why we get so fixated by sprint records. The peformance that goes beyond what you believe is possible, that cheats your retina but is also clean, is something of a Holy Grail in sport. It probably doesn't exist. The Bolt Beijing 100m and Berlin 200m were close, but I do not believe they were clean.
It is all a matter of degree, anyway. The (legal) line between performance-transforming and performance-enhancing subtsances, and the decisions made on what is or isn't acceptable, are somewhat hazy, almost arbitrary. In some ways, I have more sympathy for Lance Armstrong, who at least seemed to have a moment of searing disillusionment followed by fatal epiphany in his career, than Marion Jones, Tim Montgomery, Maurice Greene, Justin Gatlin, etc. etc., who all very deliberately and with no emotion or motivation other than greed, cheated to win and to earn. I don't think these people cared about their sport. I believe that Armstrong (once) did. I get the impression that his personality was overturned by the corruption of cycling. After that, he was driven by a ferocious cynicism and a desire to get revenge on those who cheated before he did.
I might be over-dramatising this, but there is something layered and tragic about the downfall of Armstrong that you don't get with the implosion of vacuous, one-dimensional, venal characters like my aformentioned athletes.