Regarding who is the "ideal" candidate from a writer's perspective, the matter is somewhat trickier for Umineko than a standard mystery. Why? Well, it's pretty simple; the red text.
In a novel without the meta-world attached, even if ryukishi presented four to seven scenarios where the same murders turned out differently, we'd have no proof of the various little theories that were advanced and shot down over the course of the games. Thus, the killer is "open" because he can basically pull anything he wants out of his ass. Maria is actually a midget assassin? Hey, you can't show she wasn't!
But with the red text, he limits himself as author considerably. If he says in red that Battler isn't the killer EVER, he
cannot go back on that. While that sounds inconsequential for Battler since he's the main character, anything that limits the ability of other characters to act also limits who would be an ideal culprit to the writer.
For example, say we want to suspect Rudolf. Okay, that's fine, he's got lots of traits that make him an ideal mastermind. He's clever, he's shady, he has a strong connection to Battler as his father, he has a motive (he's the only one with an heir safely off the island).
But, as we know, he dies in the First Twilight in episodes 1, 2, and 4. Allegedly anyway. For the sake of argument, let's say that over the course of the first 5 or 6 games it's confirmed that Rudolf does in fact die each time when the game suggested he did (this more or less WAS confirmed for 1 and 2, but still). Now we have a problem if we're writing this thing. If we want Rudolf to be the "true" killer, we have to explain how he manages to make people die well after his own death. There aren't very many ways to pull that off without seeming lame. So as the writer, even if I want to make Rudolf the mastermind, my hands are somewhat tied. I could possibly still pull it off, but I've made it much harder on myself.
On the other hand, the LESS information we have about someone, the more suspicious they might become. No one probably thinks Hideyoshi is the mastermind, but on the other hand nobody knows enough about him to say. Nanjo is very suspicious because we know more or less everything about him that ought to be suspicious (his surprisingly good survival record, his obvious authority as the sole medical professional, what he must know about Kinzo), but he's almost TOO suspicious, because we haven't been told ANYTHING that could mitigate him as a criminal. Indeed, the lack of information "whitewashing" Nanjo makes him immediately suspicious, but I wonder if maybe it's too suspicious. Plus "the doctor did it" is up there with "the servant did it" in murder mystery no-nos.
So basically I've arranged the people like so, from a strict author's perspective (i.e. this has nothing to do with the information about them in any particular game):
So who would
I choose as the mastermind if I were the writer?