I don't know Baldur's Gate so I can't comment on similarities or lack thereof, but Kas doesn't really want to 'become whole' again, she wants real children but she likes the power and freedom from restraints that come with being a demon. It's more 'I want to stay as I am but I also want this One Little Thing I currently can't have'.
As for 'random pointless evil shit', what Kas does may not always appear to provide her with a material advantage but everything she does is advancing her long-term goals. She wants the Champ's soul to become stronger than it already is because that means more power for the ritual she's hoping to perform with it. Narratively it's quite useful because it means Kas as an antagonist has a reason to show up, banter with us, fuck us and not try and kill us the instant we lower our guard, but also has a motive to cause problems that we have to overcome. Every challenge she throws at us that we need to become stronger to resolve means our soul gets that much more magically useful to her in the long run.