How we cite our quotes:
"As for me, it came on me suddenly, in a flash of understanding; until then I’d thought of him only under his commonplace official designation, as ‘the defendant.’" (4.6.21)
Tarrou has his finger on the classic existentialist concept of roles. Kierkegaard argued that man’s true essence is always covered up by a series of masks, societal roles that we play (like teacher, student, mother, son, friend, and so on). The difficulty lies in removing the masks to get at the humanity underneath, which Tarrou claims he has done with this criminal.
"I hardly heard what was being said; I only knew that they were set on killing that living man, and an uprush of some elemental instinct, like a wave, had swept me to his side." (4.6.21)
Tarrou later describes the world as a battle between two roles: victims, and pestilences. (He later adds a third category, "healers," but we’ll get to that in a bit.) He uses this two-pronged rubric to side with the criminal. Because they are both living, breathing men, they must be on the same side. To kill another human, then, would be to side with the pestilences. It’s as bad as being the plague. Sort of.
"In his red gown he was another man, no longer genial or good-natured; his mouth spewed out long, turgid phrases like an endless stream of snakes." (4.6.22)
Great, so while Tarrou has managed to break the criminal out of his societal role, he has now sequestered his father into one. The red robes = Kierkegaardian masks, as we discuss a bit more in "Symbols, Imagery, and Allegory."