The key here is the ability to recognize patterns. Isolated incidents, no matter how terrible are not necessarily cause for greater alarm. For instance, because some guy in Afghanistan goes nuts and kills civilians, it doesn't necessarily mean anyone else around him did anything wrong. Now, if he and a team of guys start doing this over and over? That's a pattern, that's not an isolated incident and it does bring other things into question.
The Sandusky decades long child abuse crime spree was in no way an isolated incident, nor were reports of the abuse. We know how the system works, we've seen some of the cover-up system exposed in Tennessee, we've seen it in action at FSU, so we're operating under fairly reasonable assumptions here, aside from the abundance of actual evidence. We can argue on Paterno's behalf that he could work closely with someone for decades and not know he was abusing children, that's perhaps a bit of a reach but within reason.
What seems to be hard to overlook though is that we know of multiple incidents that Joe Paterno should have, or was made aware of. We have really only one of two pictures to paint here in defense of Joe Paterno. One is that a complete bumbling fool, a dunce, an idiot of the highest order somehow came to elevate a football program to one of the most successful in college football. The other is that Joe Paterno underwent a decades long battle with senility, starting when he was in his 50s and he simply lacked the mental awareness to recall each time he was made aware of Sandusky's abuse, but managed to still coach at a high level. Those both sound ridiculous don't they?
The picture we have before us, not derived from a single isolated incident but a pattern, is of an evil man who put college football and what he wanted above the welfare of children. I would like this to not be true as much as anyone, but we're well past being able to give the benefit of a doubt. If you "accidentally" shoot one person, that's one thing, but if you keep pulling the trigger and shooting more people, sorry that just isn't an accident that's intentional. Recognize the pattern...