WARNING: The following article contains spoilers for X-Men: Unlimited Infinity Comic #11, available now on Marvel Unlimited.
A battle to return two wayward mutants to Krakoa led to Wolverine fighting one of the mutant nation's youngest residents. In X-Men: Unlimited Infinity Comic #11, by Gerry Duggan, Emilio Laiso, Rachelle Rosenberg and VC's Joe Sabino, Wolverine was going up against the Krakoan mutant known as Curse, a young girl with the ability to alter luck in such a way that she can change people's perceptions of reality and even make them injure themselves or others only through speaking.
Curse used her powers on Wolverine during an environmentally-minded attack on a refinery with her fellow young mutant Nature Girl. Even though she is a child, Curse is far from an innocent soul, and Logan punched this young but dangerous mutant to remove a very real threat.
Curse and another mutant, Nature Girl, fled from Krakoa, becoming a team to fight against people who were causing damage to the ecosystem. Unfortunately, the way they went about it was exceptionally violent, so much so that the leaders of Krakoa took a direct interest in their escapades. But when Charles Xavier could not compel them to return via telepathic conversation and was forced out of their minds by Curse's power, Wolverine was sent in to contain the situation.
Fancying themselves as "X-Men: Green," Curse and Nature Girl had been battling a team of security guards and helping Sauron maim his way through them. Taking the bigger threat at first, Wolverine attacked Sauron before Curse intervened, using her powers to force Wolverine to injure himself. Through a carefully worded suggestion, she made Wolverine stab himself with his own claws. The injury wasn't serious enough to kill him, but annoying enough to put her at the top of his hit list at the moment.
This is just one example of why Curse isn't the average child -- even at her most docile, she isn't that pleasant. When first introduced to the Marvel Universe, she was making fun of Kitty Pryde for failing to enter a Krakoan Gate. That was only a taste of the dark nature lurking beneath the surface. Curse was very much a person who enjoyed the suffering of others and found fascination and admiration in dark abilities and deeds. Even her vision of an ideal world was one of horror and fire where it seemed as if a literal Hell on Earth had come to pass. Charles Xavier even declared she was evil when she forced him out of their telepathic conversation, along with commenting on how powerful she was.
All of this, coupled with her own abilities make Curse extremely dangerous. She's been shown to make people hallucinate, change sides mid-battle, and even psychically induce a Charley Horse one of the most powerful telepaths in the universe. With powers like that, and no compunction about how she uses them, Curse could be a genuine threat to the entire world if she set her mind to it. And considering what her perfect world looks like, it's not hard to imagine her going for it.
When she began targeting Wolverine with her powers again, Logan decided to end the conversation with a sharp punch. Punching a child who could ostensibly be considered under his care wasn't the most heroic thing Wolverine has ever done, but it was necessary. Curse was a wildcard whose powers have no known limit at this point, leaving her free to operate within the greater world would have led to catastrophe.
In the end, knocking her out was the easiest solution to the problem that Curse presented. While it's not a permanent solution, there's a chance that this is the best way it could have gone. Other mutants on the X-Men team might not have had the stomach or precision needed to contain the situation. But this confirms Wolverine is definitely dark enough to punch a kid if he thought it was necessary.