On the one hand, he wanted to get away before the discovery of Mr.Glenthorpe's empty bedroom; and, on the other hand, he wished to stay at the inn long enough to suggest that he had no reason for flight, but was merely compelled to make an early departure.

The trouble and risk he took to conceal the body outside prove conclusively that he thought the pit a sufficiently safe hiding-place to retard discovery of the crime for a considerable time, and he probably thought that even when it was discovered that Mr.
Glenthorpe was missing his absence would not, at first, arouse suspicions that he had met with foul play.
"It was not as though Mr.Glenthorpe was living at home with relatives who would have immediately raised a hue and cry.

He was a lonely old man living in an inn amongst strangers, who were not likely to be interested in his goings and comings.

That suggests another alternative theory to account for the key in the door: Ronald may have left it in the door to convey the impression that Mr.Glenthorpe had gone out for an early walk.

That belief would at least gain Ronald a few hours to make good his escape from this part of the country and get away by train before any suspicions were aroused.