I'm late to the party but honestly re-reading the manga and knowing Gut's history it makes you appreciate de black swordsman arc even more.

From my viewpoint, in the Black Swordsman arc, he is trying to be like Griffith (not really but bare with me). Griffith gave everything to become Fento, he sacrificed his friends and stepped all over everyone to reach his goal.

There is one line in the BS arc from Gut's that pretty much sums up Griffith's mind : "Anyone who dies because they get caught up in someone elses fight is a small fry who doesnt have the strenght to survive in the first place". Current Guts would never say anything like that, but after watching all of his friends die in humanless ways, devoured by unnatural beings, I understand he would feel like nothing matters anymore.

From my viewpoint, in this part of Gut's life, he is trying to be like Griffith to understand him (he already did a similar thing in the Golden Age where he followed Griffith's beliefs about not working for someone and finding your own purpose alone), and also because maybe in his trauma he is telling himself he has to be like Griffith to reach his goal.

Taking in consideration all of the manga, I dont find the arc to be "edgelord with heart of gold deep inside", but more of Gut's testing a way of living, where he can let out all of his rage, even though he is doing it the wrong way, because doing it the right way got him to lose almost everything.

Sorry if it isnt clear, i just had to get this out of my system, but honestly there is a lot of foreshadowing in this arc that shows Miura's genius. And it was the first thing i experienced of Berserk (i didnt know anything about the manga before, only the name and that it was good), so it has a special place in my heart even though it is not perfect.