My issue with spiritbending is with the execution, not the concept. The Chronicles of the Avatar books give us a lot more insight into how the spirits work, and my impression is that spirits cannot process trauma or negative emotions the way humans can. They react to insults, treachery, and destruction of the environments they represent by becoming dark, chaotic, and wrathful because such things literally unbalance them. If they darken past a certain point, they can only be placated by extreme punishments against those who wronged them.

In theory, spiritbending allows you to repair these imbalances, calming the spirits to the point that it’s possible to reason with them when they’d otherwise be too wrathful to negotiate with. It never looks that way on screen, though, because most of the spirits Korra used the technique on were artificially corrupted by Unalaq and Vaatu and had no real grievances with anyone. Poor Kuruk, and likely many other past Avatars, never had that option for dealing with dark spirits and was forced to kill them to save lives.