The writers did a masterful job leading up to the finale as far as keeping options open for who Walt is going after with the machine gun.
Could be Grey Matter if the discussion of Walt's contributions to the company being only in the name wounded his pride enough.
More likely IMO, it will be the fact that Walt now knows the Blue is still out there and will be going back to shut down production for good.
I still can't decide how the confrontation between Walt and Jesse will turn out. Really hoping that Jesse somehow is the one to kill Todd though.