system
Given the following post, respond as if you're commenting on Reddit:
human
[Avatar] Is Koh still stealing faces or did Aang squash that bug?

He could pretty big threat and could cause a lot of mayhem if he chooses to.  He definitely seems like a problem that needs to be stopped before he becomes a major issue.
gpt
He's still out there. While Koh may be malevolent and has made himself the enemy of the Avatar before, he isn't outright hostile. He, like most spirits, has rules that he plays by, and these rules don't necessarily make sense to the mortal world. Aang even receives important wisdom from Koh; he just needed to play by Koh's rules to do it. 

Some spirits, like Father Glow Worm, were actually contemptuous of Koh for being so willing to help mortals who were able to pass his tests. Koh is one the of the oldest and most knowledgeable spirits in existence, seeking out his wisdom is often worth the risk. 

Likewise, Koh tends to only actively seek out a fight against people who deserve it. (Albeit, he's willing to harm innocent people to achieve this goal.) One of the reasons Kuruk's wife had her face stolen was because Koh had became angry with the Avatar for his campaign against the spirits. Stealing Ummi's face was a punishment, and Kuruk wasn't able to kill Koh in the end because he realized doing so would also permanently take Ummi away from him. 

Killing spirits is no easy task, either. The Kyoshi novels make it clear that killing even relatively minor spirits poisons your own spirit. Part of the reason Kuruk was thought to be so useless as an Avatar, and why he died so young, was because he spent most of his time hunting spirits under the belief that he was protecting the world. He kept much of this to himself, including the mental and spiritual damage it caused him. The only way to deal with spirits and not eventually die from it is to either appease them, as Aang did most of the time, banish them, as Korra learned to do, or imprison them, as Wan was able to do. 

But even so, most of the time appeasement is the easiest and most effective strategy. 

Tl;dr: Koh's still out there, and not really a threat. Koh plays by different rules to humans. Don't piss him off and he won't be a threat, unless you're trying to get something from him like Aang.