No one had witnessed the meeting between Stoner and himself.

No one had seen the blow.

No one had seen Stoner's fall.

Far better to say nothing, do nothing--far best to go away and let things take their course.

Stoner's body would be found, next day, the day after, some day--and when it was found, people would say that Stoner had been sitting on those rotten railings, and they had given way, and he had fallen--and whatever marks there were on him would be attributed to the fall down the sharp edges of the old quarry.
So Mallalieu presently went away by another route, and made his way back to Highmarket in the darkness of the evening, hiding himself behind hedges and walls until he reached his own house.