> "Karlmannlega er að farið," segir Kári.
> '(It) is gone (performed) in-a-manly-way,' says Kári.

Or even "manfully attacked" (that was a many attack)? But "manfully
done" seems to fit just as well, and whichever sense is intended, it
implies the other in this context, so maybe it isn't possible to
separate them here.

> að hér leiði af dauða eins sonar míns

It certainly looks impersonal here, with accusative 'dauða'. Here are
some more examples that might help to clarify the syntax. Where 'af'
has an explicit complement, it's in the dative, and expresses the
cause (1, 2, 7, 8). The person affected is also dative (1, 2). The
misfortune which derives from the cause is accusative (unambiguously
in 8), at least in a simple sentence.

1. en engi vandræði skal þér af mér leiða
"but you will have no problem(s) on my account"

2. og það veit eg að af henni og hennar fjölkynngi leiðir mér nokkuð illt.
"and I know that I will suffer some harm on account of her sorcery"

3. og kvað hér skyldu mikið illt af leiða
"and said much ill would result [from it]

4. en eg ætli að gott muni af leiða
"but I anticipate that good with come of it"

5. en eigi örvænt að af leiði vandræði
"but it will be no surprise if trouble(s) result"

6. "Þá er það líkara," segir Flosi, "að stórvandræði leiði af."
"It's more likely [than not] ... that great trouble(s) will result"

7. því að fleirum þykir mér sem illt leiði af honum en gott
MM & HP: "I have the impression that Mord causes more harm than good"

8. leiddi af þessu langan óþokka (masc.acc.sg.) með þeim
"long-lasting bad feeling between them resulted from this"