(256)with a bee in one’s bonnet
Some fanatic with a bee in his bonnet about justice.

「with a bee in one’s bonnet」は「おかしな考えに取りつかれて」。17世紀に書かれた詩が語源とされています(bonnetは「ボンネット帽子」)。

(257)get somewhere / as it were
“Carry on,” he said.
“Just for a minute. I felt I’d got somewhere. Got, as it were, the clue to the thing. It’s gone now. Go ahead with what you were saying.”

「get somewhere」は「目鼻がつく」の意。「as it were」は「いわば、いうなれば、言ってみれば」の意。「

(258)go into …
We went into it.

「go into …」はここでは「・・・を徹底的に調べる」。

(259)to begin with / not quite / in the dark / as to …
Now, to begin with, we’re not quite in the dark as to what happened on Soldier Island.

「to begin with」は「最初に」。「not quite」は「必ずしも・・・というわけではない」。「in the dark」は「知らない」。「as to …」は「・・・に関して」。

(260)granting that … / kill off
Granting that Armstrong was mad, what was to prevent him having killed off the others and committed suicide by throwing himself over the cliff, or perhaps while trying to swim to the mainland?

「granting that …」は「仮に・・・としても」。「kill off」は「大量に殺す」。