For one thing, LHO was completely unstable and had demonstrated that fact on more than one occasion--with his whole life, in fact. No one with any brains or connections would involve him in, or depend on him, for anything important.
Maybe not--but he was known by someone with a likely motive, and he was obviously willing and able.
Second, the various conjectures about connections and motives are worth investigating as long as there's something in the hard evidence that doesn't make sense.
One does not have to question the evidence that Oswald fired the shots--and all the shots--from the Schoolbook Depository to consider the likelihood of a conspiracy.
As a natter of fact, the Report did not state that there was no conspiracy; it simply stated that they did not find any evidence of a conspiracy, and that they did look into several possibilities, including foreign involvement, possible wrong-doing by Government agencies, allegations of connections between Oswald and Ruby, and the possibility that a third party arranged for Oswald's employment at the Schoolbook Depository.
The Warren Commission did not mention the Chicago plot with a similar intended MO; maybe only those of us living in Chicago heard about it at the time. The Warren Commission did not mention several obvious links between Oswald and Carlos Marcello, whose motives against RFK are self evident; and all of the reports about a taped Marcello confession came out almost three decades after the Commission Report.
I personally believe that Giancana (Chicago; enraged by RFK due to the crackdown on organized crime after he had pulled a lot of strings in Cook county on election day, 1960) and Marcello (New Orleans; deported by RFK and being pursued by him at the time; with subordinates who knew LHO) were in cahoots on the assassination. Others with obvious motives included James Hoffa and Santo Trafficante, both targets of RFK.
None of this contradicts the Commission Report at all, nor are the Commission's conclusions inconsistent with the possibility of a conspiracy.