In Point Blank, my initial impression was that the character of Yost was a cop trying to get Walker's cooperation in taking down the organisation. Was this Walker's impression as well?
Yost, if the imagery notion of the film is to be believed, was an "angel." He was guiding Lee Marvin's Walker character on the mission to seek revenge (and his $93,000) from the men who had wronged him. This is why Yost was seemingly omniscient about all aspects of the organization and was not injured or killed by Walker or Walker's action is in the end.
If the film's narrative is read to be a simple revenge tale (e.g. Walker is not a vengeful spirit seeking retribution for the acts of betrayal which killed him) then Yost is really "Fairfax" one the triumvirate of gangsters who are running the organization from which Walker stole the money. Yost/Fairfax uses Walker's desire for revenge to eliminate his rivals in a manner which won't allow them to suspect that he is behind these activities and cause them to seek revenge against him.
The movie can be seen by the viewer in either manner:
The final decision is left up to the viewer.