I'm also sure it wasn't easy, either. NiGHTS basically wiped out all those he called family. His brothers, sister, and father. All dead (or so he thought) because of what he did. I don't doubt that NiGHTS has often looked back at what he has done...and wished he hadn't. His brother disowned him, his partner...his only friend. Reala claimed 'There is no, NiGHTS!' because when NiGHTS traitored, it was like he had died to him. NiGHTS no longer existed to Reala because of what he did, and Reala certainly didn't see him as his brother anymore, of course. Although Reala doesn't look the part, I'm sure he was saddened by NiGHTS' decision as well.
Wow, that's almost /exactly/ what I said in an older topic a while back..About NiGHTS and Reala's relationship. oO Guess I'm not the only one who thinks that, then. Sweet! Reala's line means a lot more than most people think. ^.^
Like most of ya think, though, I say NiGHTS rebelled because he didn't like being ordered around. =3
EDiT:
NiGHTS and Reala were created by Wizeman at the same time. So, the two of them treated one another as brothers. When NiGHTS decided that being evil wasn't what he wanted and left, Reala took it badly. When Reala says "There is no NiGHTS!", I believe that he's saying that his NiGHTS, the NiGHTS he knew, no longer exists. I take too much time to think of these things, huh? Well, anyways. The two fight because Reala, in his mind, thinks that who he's fighting is not his brother, so it doesn't bother him. NiGHTS is simply a rogue 'Maren who is getting in his way. To put it simply: Reala refuses to acknowledge that his brother even exists anymore. Selph, on the other hand, is NiGHTS' mind telling him what he is. In order for NiGHTS to have a clear conscious and remain good, he must defeat Selph and prove to Reala that no matter what he does, NiGHTS is NiGHTS and will never be evil.
That's from my ancient Selph topic. XD See? Almost /exactly/ the same thought! Spiffy. X3