[k//eternal]

Note: Like the above post I made, this is also conjecture on the mastermind(s).

Spoiler:

Originally Posted by Klashikari

So what? keiichi is a complete outsider, various people also. Takano was a nurse in Irie's clinic for several years, and every villager know her.
If she is a outsider, what about tomitake who visits hinamizawa only some days each year?

there is no need to be a outsider for being the mastermind(s). the said "culprit(s)" can be a villager(s) who is one of the missing people

Well, we know it isn't Keiichi, and you're going on "square is a rectangle" logic here. The culprit has to have strong outside ties, because the MIGs come from outside (as established before here). Additionally, to want to wipe out the village, someone is probably looking at a bigger picture than just Hinamizawa itself; since Hinamizawa is so secluded, the villagers aren't likely to be thinking that way.

Consequently, the culprit is someone from outside. That narrows things down.

Now, Irie is pretty suspicious, and (IIRC) he's been associated with the MIGs before. Combine that with Takano's attitude and clearly-intentional manipulation and it's fully plausible that they're together in this conspiracy.

As for "who isn't suspicious"... well, look at it this way. Write up a list of characters, cross off the mains and the villagers, and you really don't have much left. In my experience it never happens that a mystery story has a culprit who isn't introduced as a "normal" character, because that leaves too little room for development and the culprit's motives.

So I'm pretty sure I'm right on this one, the question I have is more why, not who.