I personally I think that the lack of mention about the motive is simply due to the fact that Ryuukishi believes he made it clear enough already in the story.
Actually... he outright stated Yasu's motive in the interview with Keya already.
Quote:
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Because Umineko is „the story of a single girl who arrived at that point because she imagined an incident because of the love and madness in herself“,
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Naturally we are talking about the games here.