I divined then, Sonia, that power is only vouchsafed to the man who dares to stoop and pick it up… There is only one thing, one thing needful: one has only to dare! … I— I wanted to have the daring… and I killed her. I only wanted to have the daring, Sonia! That was the whole cause of it!
He lays out the logic behind the murder: daring as a test of worth. It’s a self-trial disguised as social philosophy. Saying it aloud exposes how thin the idea is. Sonia’s presence acts like a mirror that strips away his slogans. The slip—“I… I”—shows the theory cracking. Bravado gives way to a plea for meaning.