M
Mjölnir
Guest
I'd also argue at that point, T'Challa saw Klaw as falling under Wakandan justice, and Wakanda is shown to be super strict to outsiders. Zemo bombed an embassy elsewhere, so even if he killed T'Chaka, he still had to answer for the deaths of others as well. Further, Zemo was about to kill himself. Life was a harsher punishment than death for him.
I can see where people see this as a character regression, but I think it was consistent with T'Challa.
Just killing someone that you've taken prisoner isn't justice, and the emotional state that T'Challa was in at the moment suggests that he wasn't about justice at the time. Justice is logical, revenge is emotionally driven. He was in such an erratic mindset that he couldn't even think clearly enough to realize that killing someone in South Korea in front of a ton of people was a bad idea. He didn't even seem aware of his surroundings, while normally he's always calm and aware.
We also see in his conversation with W'Kabi that he really wants to capture Klaue, but he doesn't really want to tell W'Kabi that he's wrong to ask for revenge. So in a calm mind T'Challa was not looking to kill Klaue, but he got pushed emotionally to the point of revenge again.