Agree at some point. Wouldn't like it if it were to happen to me. Come on, left out in the dark not really knowing what is going on until to a point of death (sorta) that you realised everything was planned to have you destroyed all this time. Obviously when a lot of strange things happening and nobody is there to explain to you is going to be damn frustrating, even for myself.
Well, back to the topic. I don't think Riku was being cold to Roxas. At certain point in KH2, we can actually see that Riku was uncomfortable and didn't like how Diz is treating Roxas and Namine. You know, the train and the beach scene? He questioned Diz on why he showed the train as to make Roxas look like a lunatic and why won't he just create a simulated beach so that Roxas would have a happy vacation with his friends. These scenes for example really showed Riku actually cares for Roxas.
Also, I do like to recount the fight scene in the Days manga right before Riku turned to Ansem, he did apologise to Roxas beforehand. I'm not sure why his actions with Xion is slightly different than Roxas, maybe because Xion looked like Kairi so he was gentler perhaps? Initially he was being a jerk to Xion by fighting with her in Beast's Castle, calling her a sham and saying her Keyblade is a fake and worthless. But later he had a different tune and became nicer to her, maybe because she too questioned her existence, and all the weirds dreams she kept having made her seek answers from Riku and that in return mellowed him down a bit towards Xion.