Even though Roxas is a special nobody so the "I am emotionless" thing is not as bad as for other nobodies there are still moments where he just does not really react or follows his nobody nature to just mime an emotion (not to mention that Kingdom Hearts is an RPG and it is quite impossible to show no emotions and make it interesting so there are things like "they mime emotions" and "they are special and can partly feel emotions" which is nothing different for us who view it).

And then there is the other thing about Kingdom Hearts not always being 100% logical concerning every single aspect working together with the others. So the moment when he attacked Xion and didn't really apologize was probably not one of those which got the deepest thoughts when writing it.