Sad thing is, if Black Devil is serious about that(the killing part)... then he loses the last and only friend he has when Gramps beats and kills him.
But, with how this is, he is probably testing to see his resolve or something like that.
I'm more curious about how he harvest those carrots in the upper beam.
Also it reminds me of that Hannibal chapter with the murderer using his victims for growing Mushrooms.