I don't think Akira is being TOO hypocritical here. A little? Definitely. But not more than usual. He already killed Pon and best boi Atsushi, which was a big deal. Of course he would not kill Master, since he is not an evil vampire and he has proved to be trustworthy. Goatman is probably the biggest WTF, but I'm assuming it's because he saw him as an ally and temporarily forgot that he was a ruthless monster only an hour earlier. It's common in manga to have a "you're my enemy but I don't want to kill you because I respect you as a person" moment. However, the issue with Ken is that he hid that he was turned into a vampire and then attacked Akira's other friends. Because he is not aware of Ken's struggles, he must have seen him as a traitor. And to Akira, betrayal = worse than anything, which explains why he seemed to respect an enemy more than his childhood friend.
Is it good writing? Probably not. But I think I understand Akira's reaction. If Ken had admitted from the start that he was a vampire I'm sure Akira wouldn't have been so harsh on him. At least not immediately.