Warning, major Spoilers for games 1-5
The following is based on the assumption that this tale is a mystery and that there is a consistent culprit in every game. Also, since this is only going off of the first 5 games please try to refrain from brining up info and mysteries from the 6th, thank you. I’m willing to bet 500 cookies that this is right.
Who?: The culprits of the Umineko series are Ushiromiya George and Shannon. Whoever “murdered” Kinzo during the game would of already had too of known about his death. In this list are the servants (except Ghoda), Natsuhi, and Krauss. Because Natsuhi’s and Krauss’s innocence was proclaimed with the red truth, they cannot be the culprits. Also, it has already been said that Nanjo, Kumasawa, and Genji are not murders in the red, so I’ll say it agian; “Genji, Kumasawa, and Nanjo are not murderers!”. This leaves only Kannon and Shannon left. So here is the big piece of evidence from the fifth game, ready? The “boy from 19 years ago” somehow knew that Natsuhi’s favorite season was Autumn, but Natsuhi had never told anyone that before, no one, except Shannon. Sure she could of told someone else, but conversations where you talk about your own favorite season are rare, much less the favorite season of some 3rd party. Therefore, Shannon had to have something to do with the whole “Boy from 19 years ago” fiasco that was at the center of the murders in the 5th game.
Why?: While I’m sure a bigger sob story that will be shown at the end, the motive is that George and Shannon wanted to get rid of the Ushiromiya family so that they could marry unopposed. There determination has been subtely shown throughout the games: When Shannon broke the sacred mirror in the shrine and when George announced, when he was fighting Gaap, that he was prepared to take the whole family as his enemy and was even prepared to kill them with his own hands. It can even be said that the whole prologue to the 2nd game was Ryukushi07's way at giving the audience a motive for the 2. This idea of theirs might of started when George, sick of playing his role as the “perfect son” and the ungodly expectations put on him by his mother, complained to Shannon, the only one he found to understand him. They might of just toyed around with this idea at first, but as time passed so did their resolve. Thoughts become words, words become actions.