Neo Bahamut wrote:
Something occurs to me: Why could Unalaq not use his spiritbending technique on the portals?
That's not really adequately addressed. The series tells us he could not open the portals himself, but not why he couldn't. All we do know is that it was Wan, the First Avatar, who closed the Spirit Portals, and that only the Avatar is able to re-open them.
Thebridge14 wrote:
Why did korra have to lock the portal in the first place both needed to be open for vaatu to escape, so going into the spirit world to close the portal didn't make much sense.
We know that, but Korra didn't. As far as she knew, the risk was there as long as the Spirit Portal was open.
AvatarAang1510 wrote:
I think that if after team avatar was in republic city they should have gone in the northern portal
Quite apart from the fact it is highly, highly unlikely the Northern Portal would be left unguarded, there is one problem with that: It is closed.
In order to enter through the Northern Portal, Korra would have had to open it first - thereby giving Unalaq exactly what he wanted. Thus entering from the North to close the Southern portal would not be an option.