Superman has been around for 75 years. He's a comic book adapted presence. There's been hundreds on top of hundreds of comic books ranging from each era to where it's impossible to know his entire back-story word for word. There isn't one take on Superman. There isn't one way. There's been plenty of character personalty shifts and each era has them with hundreds of different writers.
The fans simply remember their favorite incarnation of Superman, and claim it to be the ONLY way when in reality, it isn't. It's a close-minded approach that hurts the character in new adaptions such as Man of Steel.
As for the big spoiler mentioned, to me, Superman isn't about making the right decisions ALL the time because he simply, can't. It's how he responds to big decisions or maybe mistakes that make him Superman to me. There's more to him than meets the eye.
Some fans should really allow change to their favorite adaption and enjoy the story of what makes him tick rather than being upset with a decision made that doesn't connect with their view of a 75 year old character.
As I said, it's easy for me to say. I look at Supes to where I'm OPEN to the fact to add different dimensions to the character and be open minded. He'll always be Superman at the core no matter what but I don't expect EVERY action to be flawless.