If the red slayer think he slays,
Or if the slain think he is slain,
They know not well the subtle ways
I keep, and pass, and turn again.
Far or forgot to me is near;
Shadow and sunlight are the same;
The vanished gods to me appear;
And one to me are shame and fame.
They reckon ill who leave me out;
When me they fly, I am the wings;
I am the doubter and the doubt;
And I the hymn the Brahmin sings.
If Brahma is everyone and everything, separation from everyone and everything would make “them” reckon you ill. 
 He combines in “reckon ill” both “wishing for illness upon” as well as “reckon ill of mind”