If hes arrested before the evidence is assembled and a case built, he walks.
That is just silly. Most criminals are arrested before evidence is assembled and a case built. For instance, the deceased was possibly involved in fraudulent activity. Do you really think evidence was assembled and a case built before first contact?
Obviously, the facts were different in that case, but the point is that if the prosecutor doesn't have a solid case, a later acquittal can be just as destructive as the protests at the beginning.
They're just trying to make sure their case is air tight. Now, everybody here clamoring for an arrest, a conviction, and a long prison sentence appear to know more than the investigators. What evidence does the public have that the prosecutor is missing?