Didn't Lulu say in Kilika during the sending that if you are not sent then you become a fiend because of your hate for the living. In Aurons case he was fulfilling a promise. Maester Mika was a wise leader and existed only to guide spira using outdated methods (ie; the final summoning) so he was basicaly a good guy. Semour was insane and under the alusion he would heal spira by killing them, a twisted sort of love for the people. Also Semours father Jyscal was trying to warn of semours plan. and as for Auron aging that was probably just to keep people from wandering why he hadn't aged.