Again with the whole motivation thing, you'd think that Nobodies wouldn't care about not being "whole".

For that quote I can respond as in it's probably an instinctive thing like a heartless' inability to resist killing.

As with Axel and Roxas emotions, I think Roxas' is explainable as he was put in a virtual world by Ansem and had to interact with the virtual people there so he had to attach some of the personality he had to live in that life, which he got from Sora since he is half of what he is unlike the others who do not have their human dopplegangers anymore.