Really I would say the word choice is important here. It really wasn’t a “murder” at all since it was an extension of ritual combat that is agreed upon. Both parties knew that this particular fight was for more than just the throne and ends with someone’s demise. Less “murder”, more standard combat 👍🏻
Plus, T’Challa’s blade would have killed Erik regardless unless he had accepted medical assistance from Panther. He would die whether he pulled the blade out or not, but he expedited his own death by “pulling the plug” if you will.
Basically, I would just say that T’Challa won the fight 😎