The lumbering corpse doesn't really see David, all he sees is: blood, life, meat.
But David sees the corpse who used to be Justin from #10B; he knows that apartment up and down and he knows Justin enough that tears well up as he recognizes what used to be him under the peeling putrid skin and milky pupils.
The thing corners him behind a Carvel shop; now half blown up from some riot or another.
David's got a bad leg, which he injured jumping a third story roof to get away from an endless line of corpses, and there's nowhere to go, so he falls to his knees in the narrowing gap between the Carvel and the Kinko's, praying.
As the thing that is not Justin rips into his lower bowel with its decimated grey fingers and he screams at no one, he wonders if some part of the corpse remembers that morning when Justin walked into #10B to see his wife up against the refrigerator, mouth parted and wet, with David's hand shoved down into her jeans.