I thought about that too. I don't think the first half of this season was particularly well thought out, he could have at least waited until they were both at the North pole so she wouldn't find out. I can think of only one reason - that he is a megalomaniac who really wants to get another one over his brother. But we…

I think he had to invade the South to stop Korra from closing the portal once she figured everything out. She wouldn't have had to go to the spirit world if she could have just gone to the south pole.