Whats wrong with juror 7 saying "If the kid didnt kill him, who did?"

Most of men pretty much decide the Kid was the murderer. Number nine (I think casts doubt) on this which upsets the other jurors because they want this trial finished.

They aren't suppose to base his guilt on knowing who else might have done it. They are suppose to base their verdict on if the prosecution proved "beyond a reasonable doubt" that HE did it. If they have doubts he did it the verdict has to be NOT GUILTY.