as i'm re-reading the duelist manga, i think my headcanon is that all of the "Yamis" are some combination of Zorc as well as their hosts/original souls. The "evil intelligence" of the Millennium Items that Pegasus references is the fragments of Zorc sealed in the Items; split into pieces, but still corruptive and influencing (also, the dead souls of the people used to make the Items in the first place, given direction for their wild vengeance by Zorc's overwhelming mental presence).
Yami no Yugi is a combination of Zorc and Atem, which is why he starts out a bit more violent than the past version of Atem that we see before. All of Atem's soul was in the Puzzle, so it's like 100% of Atem's soul + 14% of Zorc's. Luckily, he also has Yugi's influence and then interactions with the rest of the gang to temper those feelings and bring him more in line with his past life self. He's not quite Atem anymore, but he's not very much of Zorc either.
Yami no Marik is also about 14% Zorc, but remaining is all Marik's mental distress. I headcanon that Marik has DID, and just one of his alters happened to get mixed up with Zorc's influence and become a lot more murdery because of it. There's probably also threads of previous Millennium Rod bearers that got tangled up with that fragment of Zorc's soul as well, so there's not just Zorc's appetite for destruction or Marik's own mental fragility, but also the pain and suffering of generations of Tomb Keepers all mixed up in it as well into this conglomerate entity that's mostly made up of hate and revenge.
The other Millennium Bearers manage to avoid getting too much Zorc for other reasons, mostly because they were older when they took on the items and had more of their identity solidified that Zorc couldn't get a big foothold in them, so they show some signs of that murderous intent, but not enough to completely develop a Yami persona.
In Ishizu's case, the foresight that the Tauk gave her helped her to manage the pieces of Zorc that came with the item because she was mentally prepared for it, and she was older when she took it on so she had developed mental fortitude.
Shadi can take on two items without going completely crazy because his sect of the Tomb Keepers are more aware of the entity in the Millennium Items and he probably trained for years to develop the mental walls necessary to wield the items - which probably contributes to why he is a lot more emotionless than other characters and can seem cold, especially towards taking lives. He might not be as dangerous as other pieces of Zorc, but it comes with its own mental scars as well.
And then Pegasus, again, a little older and more secure in his identity, but also he's the first one to reference the "evil intelligence," so it's likely he's felt some of those dangerous, murderous and destructive impulses since he got the Eye - which combined with his grief and his desperation to see his wife again somehow, leads him to becoming a pretty ruthless person.
And finally, Yami no Bakura. Bakura is almost certainly a combination of Thief King Bakura and Zorc, and I think a lot more of Zorc coalesced in him over time because Zorc and TKB were so closely associated even before Zorc was fragmented and destroyed, so Yami no Bakura is definitely the Most Zorc of any of the Millennium Item bearers. He's not fully Zorc because he's also Thief King Bakura and just a little bit of Ryo mixed in from their interactions over time, and because we see that Bakura himself is not always exactly sure of what he needs to be doing to achieve his goals. His memories are fragmented and mixed up, just like Atem's.
TL;DR, the so-called "evil intelligence" of the Millennium Items is mostly Zorc+the anger of the people who were made into the Items that he controls and wields, sealed inside them and split across them, influencing the Items' bearers over time.
this also ties into my theory of yugioh soul mechanics in that i think that souls in yugioh are like plants but i won't get into that right now














