The thing was physically possible.

If Bellingham had really been in the study when Hurst came home, the murder could have been committed--by appropriate means--and the body temporarily concealed in the cupboard or elsewhere.

But, although possible, it was not at all probable.

There was no real opportunity.

The risk and the subsequent difficulties would be very great; there was not a particle of positive evidence that a murder had occurred; and the conduct of Hurst in immediately leaving the house in possession of the servants is quite inconsistent with the supposition that there was a body concealed in it.
So that, while it is almost impossible to believe that John Bellingham left the house of his own accord, it is equally difficult to believe that he did not leave it.
"But there is a third possibility, which, strange to say, no one seems to have suggested.