IMHO from an economic pov the only true merit of Marxist analysis is to realize the inner historical trend of capitalist economies to monopoly/oligopoly.To this regard:
on one hand the diminishing rate of return of capital assets provided by Marx as an explanation for this process is empirically and historically problematic if not completely untrue (technical progress is the clue here).
On the other hand I've never fully understood which real role/responsability (if any) the state/central authorities has in this gradual decline of free market competition through time (at least since the industrial revolution).
It could be intrinsic to free market capitalism even without state intervention and meddling.
Taking to the extreme the latter, the 'leviathan' could be the natural (by)product of an initial state of 'free market' in security and defence given the natural tendency of men to violence and the self reinforcing mechanism of centralization of means to commit it. See as a reference the european transition from feudal small and weak states to large absolute monarchies thanks to warfare.