Roku and Sozin were well known to be very close childhood friends. Sozin, at least early on, did seem to have a vaguely humanitarian agenda, and Roku knew this. Roku of course was opposed enough to Sozin's imperial ambition to destroy his palace. After this it seems that Roku didn't have a lot of direct contact with Sozin and may have been unaware, through a combination of lack of information and wishful thinking, that Sozin's attitude was hardening. He would certainly have been unaware of any plans (which probably didn't exist yet) for a genocide of the Air Nomads, and, since no previous nation had set up a colonial empire of the sort that the Fire Nation did, he wouldn't have known how bad things could really go.

Most of us aren't in a position where our best friend might, but also might not, be planning to make war on the whole world, so it's hard to understand Roku's actions. From the little that Aang said or wrote about his contact with Avatar Roku it seems that Roku came to understand the grave mistakes he made.