The notion of "reasonable doubt" means that there is no plausible alternative scenario which would explain the facts. For example, suppose that someone were accused of robbing a convenience store and the only reasons the police fingered the suspect are (1) he was found within a few blocks of the store, and (2) he resembled the person on the security video. In such a case, the defendant could suggest to the jury that the store was robbed by someone who looked somewhat like the defendant (but wasn't him), and that the defendant was in the neighborhood for some other reason.
In the Scott Peterson case, there are many aspects of Scott Peterson's behavior for which no plausible explanation comes to mind other than him having either killed his wife or otherwise been involved with her death. Although there might have been some plausible scenarios where Scott Peterson's involvement would not constitute murder, it would have been up to Mr. Peterson and his attorney to suggest one. As far as I know, they have not.
I would add in closing, by the way, that if a person has involvement with another's death that falls short of murder, but works to conceal or destroy evidence showing the specifics of such involvement, it is the fault of that person, rather than the state, if he is convicted of murder.
It is NOT the defendent's responsibility to dream up and supply an alternative. It is the state's responsibility to prove its case.
You seem willing to execute someone for a murder they didn't commit when all they're guilty of is a simple lapse of judgement (and a potentially FAR less serious crime).