It may simply have been something that might have been better written as
"and about the hanging of the negro in the Chincha Islands".  Had he written
it that way, Wakeman could have been seen either as the hanger or the
avenger.  It may be instructive that he uses the word "his" in front of all
the other items in his list in which Wakeman could have been a protagonist.

But I don't know the right answer, as usual.

Carl