Tone in Crime and Punishment
By "Later on..." at the start of this paragraph, Dostoevsky relates the thoughts that Raskolnikov has much later in his life regarding this moment in time. He realizes that there was no reason for him to go home through the Hay Market, and he somewhat attributes this choice to it being predestined for him. This creates a sense of foreboding, as if the crime he intends to commit is outside of his control.
Dostoevsky portrays Raskolnikov in an incredibly ironic situation with these two men on either side of the pawnbroker's door. Raskolnikov is, just like Alyona Ivanovna was, hiding and pretending not to be home. This irony compounds on the scene to create an elevated sense of risk and suspense.
The description of Porfiry's facial expressions above are either being related to us through Raskolnikov's point of view or through Dostoevsky as an omniscient narrator. However, Dostoevsky deliberates uses this ambiguity to make it unclear for the readers to know if Porfiry actually does have knowledge of Raskolnikov's crime at this point, which adds to the tension of the scene.
Dostoevsky personally introduces his own voice into the narrative in this section. By choosing to do this and end the chapter in this way, he creates a tragic but hopeful end to this part of the story with Razumihin finally coming to an implicit understanding with Raskolnikov and resolving to do his utmost to help Dounia and Pulcheria Alexandrovna.