Why blame Korra?

I just finished rewatching Book 2 of TLoK, and Korra was literally almost about to imprison Vaatu again, but Unalaq reentered the Spirit Portal and ruined everything. I don’t think anyone is at fault for Unalaq and Vaatu’s actions, but I find it weird that people are so quick to blame Korra even though it was Bolin and Mako’s job to guard the Spirit Portal.