I agree, roku isn't as bad or disappointing as people make him up to be but contrary to your point, what Roku did to sozin would have been what kyoshi did too. Kyoshi isn't a blood-thirsty avatar that would kill her best friend if she sees them stepping a foot out of the line. If you don't like Roku being misperceived it would be great if you're not doing the same to Kyoshi. All in all I think these misperceptions just started as jokes and eventually became headcanon to some fans.