The blood trickling on the floor off the handle of the knife had turned it into an extremely plain case of murder.

Mrs Verloc, who always refrained from looking deep into things, was compelled to look into the very bottom of this thing.

She saw there no haunting face, no reproachful shade, no vision of remorse, no sort of ideal conception.

She saw there an object.

That object was the gallows.
Mrs Verloc was afraid of the gallows.
She was terrified of them ideally.