This is intriguing & mysterious.  We don't know how Knight died.  We assume foul play but don't know it.  I agree with Susan that I've never seen any such version from tradition and feel it a bit unlikely that one would have Lady kill him.

This is, I believe, a very important ballad as a thinking/teaching piece.  The main theme is generally retained but the treatment in Scotland (cynicism - or, as I take it, as life really is), England (romance) and America (travesty) just fascinates me.  It's incidentally generally a good song, too.

On the point of the question raised, one line in "Twa Corbies" always disturbs me.

An' naebody kens that he lies there, o
But his hawk and his hound and his lady fair, o.

How does she know?  Why doesn't she tell anyone?  How can she leave his body to the most terrible fate - flesh eaten by animals, lying unburied, unsolemnized "for evermair?"

In England, if she's involved, she clearly loves him tenderly & carries him home.

In the US she not mentioned.  Here, I've learned a new thing.  I learned it & first heard it from McCurdy.  He sang (for no less than Mac Leech)

There lies a knight on yonder plain,
Who by some cruel butcher was slain.

But in all the versions I see in Bronson (and sometimes by McCurdy), it reads 'There lies a horse.'
I don't know if I should make anything of that or not.

In spite of the song's popularity today, it may be recalled that neither Sharp nor Greig ever found a version and that the current (brilliant!) tune for "Corbies" is a recent, not traditional one.  The
traditional one just wasn't all that good.  This, obviously, accounts for the scarcity of a fine text in tradition.

One could easily weave a plausible plot by collating the three versions, but that's cheating & "improving."  No, just one of life's Great Mysteries.