And so, when the time came, he would only have to go quietly into the kitchen and to take the axe, and an hour later (when everything was over) go in and put it back again.

But these were doubtful points.
Supposing he returned an hour later to put it back, and Nastasya had come back and was on the spot.

He would of course have to go by and wait till she went out again.

But supposing she were in the meantime to miss the axe, look for it, make an outcry--that would mean suspicion or at least grounds for suspicion.
But those were all trifles which he had not even begun to consider, and indeed he had no time.

He was thinking of the chief point, and put off trifling details, until _he could believe in it all_.