It's pretty obvious that gat's is becomeing a demon or is atleast making one thanks to his massive emotions and the armor.
It's not that Guts is becoming a demon, but that he's moving away from his muderous tendancies and The Beast of Darkness gained its own ego and wants him to move back.
As Zephos said earlier, our current Guts is in a sort of transition stage; the original Guts had "innocence" so to say, that is, he had little knowledge of the supernatural and his life was very much human. That would be Golden Age "medieval" Berserk.
After Guts had basically everything that mattered to him taken away, he was consumed with hatred and went off on a lonely journey into a now nightmarish world. This Guts was the most "evil" or "badarse" Guts as people know him; apathetic, instictive, merciless and powerful. What made Guts exceptional among characters simmilar to him was that he still had the shining "human" element to his character; Puck's presence was likely what brought this out. We don't see Guts slaughtering villages and raping women; in spite of his tendancies he was still above all that demonic stuff, which made him a very interesting anti-hero. This is "Retribution" saga.
And as we all know, Guts is starting to regain a lot of his humanity again what with the growth of his new "party". Such is "Millennium Falcon".
anywyas, the "main bad guy" is obviousally Griffith, But the Supposed "Demon King" was only a haligram ((If you will)) That can attack physical things.
Idea of Evil is unkillable; it has no concept of death and lies within the underworld itself. The Dragon Slayer is potent enough now to slay the God Hand within the Nexus (where Behelit-Ceremonies occur) but probably not in their base plane of the Vortex.
The only way Idea of Evil can fall is if humanity itself desires its death, which isn't likely; we only heard about the "evil" side of Berserk first because that is the most powerful. Beings like the Elemental Kings are basically forgotten in this "dark age", even though there are always forces of good to contrast those of evil.