"Is that what people would consider a noble self-sacrifice? To throw away your life and what you can do because of your metaphorical tunnel vision? Don't be stupid, Riku, I think we both know that you're at least marginally smarter than that."

It's probably not encouragement, at least not in the conventional sense. No doubt Xemnas has his own motives and reasons for saying the words, but what they are, he isn't letting on.

"At the very least think of these people you call friends. I expect that would be angry at you, rather than poised to applaud your martyrdom. And... what of the witch? Would you abandon her so easily? I do not think that you would."