What I found on the Fandom Wiki here about W'Kabi from Black Panther:
Having lost his faith in his king due to his failure to capture Ulysses Klaue, the man who was responsible for the death of his parents, W'Kabi put his support in Erik Killmonger, allowing him to take the throne of Wakanda and seemingly kill T'Challa as a result.
That doesn't make sense though. Yes, Killmonger confronted W'Kabi with Klaue's corpse but this should not be the persuasion that made him to go against his best friend T'Challa. I mean, although Killmonger is a half Wakandian, but he is not eligible to be ruler of Wakanda, neither his personality and the way he rules. How could he put Wakanda in risk just because of his personal grievance?