It is kinda odd how Kira talk so much more to his victims than any other people. Like he only really feels comfortable talking around those that he will soon render unable to harm him.
Well I don’t think it’s so unusual; you more-or-less explained it yourself right there!
In addition though, I figure it’s a power thing. He does a bit of banter beforehand, yes, but it usually leads back in to humiliating the target, making them feel helpless, or just making them feel lesser in general. He is genuinely chatty in these moments, but it’s not like he’s really looking for a conversation or anything so two-sided. Even with Koichi, realize that he never really responds to what Koichi says (with the exception of addressing things that are a direct threat to him) in lieu of talking over him or mocking him. He’s always the one leading the topic at hand, and if he wants a response from a person, it’s one that he has forced from them.
It’s only after he’s been put into a vulnerable position that Kira is forced to converse.
It’s actually really interesting.