I think it's Kou. Here's why:
In his first message, he starts off like this:
I don't know, Larkin may have been the first to point fingers but I don't think that qualifies him as the killer.
This makes him sound like he's trying to put thought into this, and not just go by first instinct. The killer would probably not be the first to post, as it would draw too much attention. But then he says:
I'm tempted to say Mister_Prophet myself, however. The emoticon just makes me antsy. Also, why would he be interested in adding numbers to someone else?
WHOA THERE. He just threw that logical reasoning out the door and acted on someone's avatar, which is even more ridiculous than going by first post. Trying to make a scapegoat, most likely, because he knows that as long as the attention is on someone else, it won't fall on him. Mr. Prophet just happened to have that serial-killer looking avatar, which Kou figured would work against him, first impressions being what they are.
So next thing you know, Kou knocks off someone who accused Mr. Prophet, to make it look like Mr. Prophet is trying to kill off the people who think it's him.
Now this is all in theory. I am not 100 percent sure that it is Kou. However, the most likely people to be the killer at this moment are Kou and anyone in the game who hasn't posted yet. This game can be easily won over the internet by simply laying back, killing random people and letting everyone else fight it out. If no one accuses you at the start of the game, there's no reason to post to defend yourself, and after that first round most people will only vote for people that have already posted. Out of sight, out of mind.
The least likely person to be the killer at this moment is Mr. Prophet. In this game, when accused, the killer will try and appeal to logic to shift suspicion to someone else, all the while trying not to look like they're shifting suspicion away from them. Mr. Prophet quite blatantly states that he is trying to shift suspicion away from himself. The real killer is more likely to be more subtle at it.
.
.
.
.
Can you tell I'm studying to become a psychiatrist?