Why does he not die and end this suffering?
He groaned aloud unconsciously and started with affright at the sound of his own voice.

Was he going mad?
Terrified by the thought he turned away and ran towards his house repeating to himself, I am not going mad; of course not, no, no, no! He tried to keep a firm hold of the idea.
Not mad, not mad.

He stumbled as he ran blindly up the steps repeating fast and ever faster those words wherein seemed to lie his salvation.

He saw Nina standing there, and wished to say something to her, but could not remember what, in his extreme anxiety not to forget that he was not going mad, which he still kept repeating mentally as he ran round the table, till he stumbled against one of the arm-chairs and dropped into it exhausted.

He sat staring wildly at Nina, still assuring himself mentally of his own sanity and wondering why the girl shrank from him in open-eyed alarm.