What is the big difference between regular Riku and black mask Riku? Like what changed in his story to change his motivations and methods?

Thanks for the ask :]

So Black Mask AU (or Black Captain/Dark Captain I haven't entirely settled on a name) is a new game plus AU and that's important to explain the changes in his motivations!

The AU starts exactly the same as Captain, Riku moves to Tokyo, Hatanaka is a piece of shit to him, he meets the thieves and they start investigating her palace. However this time round, something in the palace goes wrong and the thieves end up failing the deadline. And failing the deadline for Hatanaka's palace... results in Riku's death :] (putting it shortly Riku is more useful to her dead than alive so she has him killed)

That should have been the end of him, however he wakes up immediately after getting shot. After being understandably disorientated he realizes he's somehow been transported to 2/3 years prior to the events of Captain (a couple of months before he was due to move to Tokyo) and he has all his memories intact, right up until the point he was shot.

This understandably messes him up a lot because he went through hell and on top of that he remembers everything he learnt during Hatanaka's palace (I won't go into much of that as its spoilers for the main AU). He also decides to go into the Metaverse in Port Island because he still has access to it, and its here he awakens to his black mask persona (Mordred who I ADORE lmao) and where he first kills someone (albeit by accident).

As for his motivations someone put it really well once, spite in general is a huge motivator for Riku in both AUs! Black Mask Riku basically takes the idea of him being motivated by spite to a whole new level, as his main goal in the AU is to get revenge on those who wronged him and who caused his death in the original timeline. He's here and alive in spite of these people and he is sure as hell going to make their life hell.