And in this assumption I'd have to say that you're incorrect.
T'Challa's promise to kill Namor had nothing to do with his ancetors and more to do with Namor's Phoenix Force enabled actions against Wakanda during AvX and herein lies the problem.
Jonathan Hickman inherited the messy fallout from the aformentioned event that in turn, led to a number of conceptual problems that have plagued his New Aveners saga rihgt from the very begining.
Prior to AvX, Namor and T'Challa had always been allies with zero threat of war between their repective Kingdoms but all this changed in much the same way as T'Challa's relationship with Doom changed in the aftermath of Doomwar written by Jonathan Maberry. (which to be fair, had it's origins in the final issues of the Black Panther's solo book penned by Reginald Hudlin before Maberry took over.)
Heading into Hickman's New Avengers, we now had a Namor portrayed wildly out of character surrounded by other well known and iconic Marvel maintays who over a period of time, were equally shown acting in extremely illogical manners at wild variance to their already well established respective canonical continuities within the 616 MU.
The combined intellectual genius of Reed, T'Challa, Stark and Strange (prior to the arrival of McCoy and Banner) were stumped by the Incursion threat and enigmatic obsfucation of the Black Swan and we the readers were presented with the specatacle of the Illuminati chasing shadows rather than reaching out to other powerful minds within the 616 MU for assistance.
And in the midst of all this, the conflict between Atlantis and Wakanda that began during AvX, was still raging unabated.
Written in character, T'Challa would never have invited Namor into Wakanda under the existing circumtances and he certainly wouldn't have gone on to allow the building and stockpiling of WMD's within Wakanda's borders either but we're talking about a story that intrinsically has had to rely on PIS to move forward with all characters concerned (at least from the supposedly heroic side of the equation acting like morons.)
The saga was flawed right from the very beginning and to pretend otherwise is to engage in pointless folly.
So far, the only character that Jonathan Hickman has written properly in NA is this guy right here....
But as for the other major players in this saga, he hasn't done a particularly good job of capturing the quintissential spark that makes each of them special.
At this point, I could care less who gets to kill who.
I just want this morbid tale to reach its conclusion.