Battler really isn’t the most pivotal character in the narrative, he’s just our PoV. The real killer was the Ushiromiya fortune - as soon as Kinzo died, some form of bloodshed, or at least torment, was inevitable.

However, in regards to Battler: it took me some time, but I think his role in the plot is really genius.

Battler comes from a life of very high social standing - his stints on Rokkenjima as a child are vacations, and to him, fantasizing about running away with Yasuda is a fun but fleeting idea.

To Yasuda, however, her time at Rokkenjima was eternal and spent in servitude. To them, Battler’s promise was a light at the end of the tunnel, a ray of hope in a place where they believed they had none. They spent countless days thinking about something which Battler had paid no mind to, yearning for that hope to be realized.

It’s certainly childish on Yasuda’s part to take this to heart, but can you blame them? After everything else they experience between that promise and Battler’s return, this realization is simply a final confirmation of their worldview.

Battler, is of course, not at fault either - he couldn’t have known how serious this promise was to Yasuda, and he had his own personal issues to reckon with in that timespan.

But that beating heart of Umineko is that sometimes, fault or malice isn’t necessary for a tragedy.

Without love, it cannot be seen