I don't think Spurrier was a bad decision - letting Spurrier be both head coach and GM was a bad decision. That's problematic with a lot of high-profile head coaches in the NFL. I think Spurrier could've succeeded at the pro level, but his stubbornness to turn the Redskins into a retirement home for washed up Florida Gators proved to be his undoing.
If I had to cite dumb decisions that strictly fall to Snyder, it's stuff like charging fans for attending training camp, the hiring and firing of Marty Schottenheimer, firing Norv Turner while still in the playoff hunt, bringing back Vinny Cerratto, Jeff George, etc.
In terms of Snyder's amount of personal liability for the failings of Gibbs, it's kind of hard to pin it all on him. You have a team, while under performing, that is still in the playoff hunt. And less then two season ago, he put them back into the playoffs and actually won a post season game. Not to mention that up until recently, Gibbs might have well been sainted in the Washington D.C. metropolitan area. It's akin to firing Joe Paterno - can you really do that to a man who has done so much for an organization?