Why are the bullies (Seth Devlin and Ricky) in Grandma Death's (Roberta Sparrow's) house?

from Movie FAQ – Donnie Darko on Rap Genius

Meaning

Earlier in the film Rose Darko refers to Roberta Sparrow having a gem collection, so we assume that this is a reason for them being there.

Of course they needed to be there as part of the ensurance trap for Donnie, so they are also acting as the Manipulated Living; Ricky is a pawn of fate, as is Seth.

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