Well, we know it's an inside job because the antagonists actually know who the agents are and where to find them. Whether it's a rogue agent or someone else that we haven's seen who T'Challa trusts, it's someone who is Wakandan.
I would like it to be S'Yan who would have probably the best motivation besides Hunter. If they wanted to hurt T'Challa/Wakanda, they could have just simply exposed the spies to the rest of the world. This would accomplish giving Wakanda a political nightmare to deal with.
But killing the spies makes it something personal. Someone really wants to hurt T'Challa and put him through a lot. It's got to be someone who knows where the secrets are, where the bodies are buried and how the King of Wakanda would operate. It's gotta be family, someone who was once king and someone who was a former BP.
S'Yan fits the bill.