Let me explain the thinking I put into my Death Note themed game.
I wanted it to be more of a "back to basics" old fashioned logic and policing type game where the cop didn't just roleclaim right away and the doctor secretly healed him. That would have been too unfair for the mafia.
So thus, as Zexen assumed, I based it off the classic 12 player game with 3 mafia, 1 cop and 1 doctor. Due to the difficulty of it being a "dark game," I figured it would help Kira more than the town, so I added a 13th player to tip it more in favor of the town.
Again, it would have been too easy to roleclaim cop and make the doctor heal him every round, so I turned Kira essentially into a Godfather role, making him appear innocent. This also served to even out the game; you couldn't trust the cop's investigations 100%, but at the same time, if Kira ever made any sort of logical error and got arrested, the game would end right away. This sort of raised the stakes a little without throwing off the balance.
You guys had Zexen pinned on day 3 or so, but then you guys chickened out and didn't go through with your plan on voting him off. Without any townie to take the lead, you went back to random voting, which was your downfall. Since at that point 3 kiras made up 40%, they voted as a bloc and ended the game.
I was hoping Ninjablade would speak up and roleclaim, since he knew that Gnizmo wasn't a kira because he saved him during the 3rd night. But he didn't have the balls to roleclaim and say anything so he just let the game slip through his hands.
In this game, the doctor had the ability to save himself, so Ninjablade could have revealed this information at no harm to himself, really.
Kamal ended up dying early because he was an idiot and asked if he could refer to the doctor as Watari. If that didn't tip off Kira, I don't know what would have. Also, I never disclosed to ANYONE that there was a doctor to begin with, short of PMing Ninjablade at the beginning.
Insomiac basically had the game figured out in just one day of play. He noticed that Son1x tended to bandwagon vote on all the mislynches, and called him out on it on a couple occasions. He was then quickly deathnoted and none of your townies dared to press Son1x on it.