My favorite interpretation - possible SPOILERS
Maybe spoilers ahead...
and perhaps this has already been discussed in another thread....
Yost/Fairfax is the Uber "Boss" who assists and uses Walker to mow down the criminals who loosely form an organization with its own hierarchy. As Brewster lies dying he identifies Yost as Fairfax...one of the men Walker is presumably after for revenge. However, Fairfax/Yost is smart enough to exploit Walker's need for revenge and uses him to track the rest of the gang. Yost/Fairfax's plan was probably to kill Walker as well as soon as the rest of the competition was mowed down. However, Yost/Fairfax miscalculated one detail about Walker. Yost assumed that Walker would be greedy enough for the money and tries to bait him with it several times. When Walker realizes that Yost and Fairfax are the same person, he realizes that he had been used and knows that he must protect himself from the new threat since he has now helped make Yost/Fairfax a very powerful figure in the absence of other competition.
I would be interested in what other viewers feel about this theory.