Honestly, Ken should have been honest from the start. After all, back in chapter 131, when Atsushi revealed himself to be a vampire and outright stated to work for Miyabi, Akira was still believing in him, and Ken was there to see it. There is no way Akira would have attacked Ken just because he is a vampire.
But now? Now it looks like Ken was hiding it from them to wait for an opportunity to strike. Akira is surely gonna feel betrayed about this.