It is quite natural to seek the motive of a crime, assuming that once you discover the motive you will discover the perpetrator(s).
Motives can help but are not necessary to solving crimes. Simplistically, the motive was, somebody wanted Sherman's dead. Why and for what reason or purpose is intellectually gratifying to know, but not that important to solving the who did it.
For a moment let us suppose the Walking Man, was the sole assailant. If he was hired by someone, or had his own reasons for wanting the Sherman's dead does make much difference at this point to identifying who he is. Until we know who he is, this crime will not be solved.
Since the Walking Man has not been publicly identified, chances are are slim at this point in time he will ever be, unless someone comes forward and talks to the Police. Sadly the $35 million reward has not brought forward valid information.
MOO