1) He didn't force anyone to do anything, as you said, they corrupted themselves. He knew many of them cheered as he suffered for the sake of their own glee and satisfaction, something they probably did to many innocent people. He wanted to see if that was just a momentary lapse in morality or a deep seated flaw. He gave them an environment to show who they are. The fact many didn't change shows they had the chance to stay pure. He couldn't have left the truly decent people w>th the ones that would take to opportunity to harm them when a greater force wasn't around.
2) He chose to separate the decent people, not for his own satisfaction, but to take them away from the danger down on the ground. He all but said that himself. If I remember correctly, he planned to send them back after the danger more or less subsided. He didn't foresee the gods attacking them since they usually see humans as nothing more than praise-cattle. He didn't realise they target them just to screw with him. In fact most of the gods would have left them be, it took a supremely insecure god to stoop so low as to do that.