Guts probrably battles the Beast daily if your talking mentally. It seems his armor has one thing in mind. Death. Im sure guts fights the urge to swing is sword all the time.
what i find odd is that the beast and the Guts' beast-state are VERY different from one another.
the dog-like shadowy apparition that torments Guts is very articulate and thus comes off as intelligent, whereas when Guts' succumbs, he turns into a mindless...well, BEAST.
I believe that the apparition is not Guts' subconscious playing tricks on him but rather an agent meant to bring out the worst in Guts and permanently turn him into a demonic berserker.