I'm not sure letdown is the right word. I think Barry Bonds was always a talented player. But he is also a terrible human being who basically told the whole world that if they didn't like him, they could lick off his cream.
Did people know he took steroids? Of course they did. It's not a question of that. He's never actually been caught, so they can't do anything about it. The issue is the pure horror of baseball fans who see 755 as a superhuman achievement, accomplished by a man who was not only a model citizen, but also faced tremendous racial prejudice (and even received death threats) which served only to fuel his desire. Henry Aaron was a tremendous ballplayer and a tremendous man, and now his name has to sit forever in the record books below that of Barry Lamar Bonds. Kind of rubs you the wrong way, doesn't it?
I should temper this with the comment that steroid use can't make you good, they can only make you better. Jason Grimsley took steroids, but you don't hear his name being offered up for best pitcher ever. Bonds was destined for the Hall of Fame even before his steroid use. If Ken Griffey Jr. had taken steroids, he might have over 1000 home runs by now. That's the true crime here. It robs people like Griffey, Jim Thome, and Frank Thomas of true glory, because as great power hitters, they never had to consider an extra boost. Now their achievements are discounted as the 500 home run plateau is cheapened and the bar for immortality is set higher, thanks to the actions of Bonds, McGwire, and Sosa (just to name a few).