It was all simple enough.

To get Paolo into the workshop alone--a blow--the concealment of the dead body until night--then the short three hundred yards with the hand-cart--it seemed very practicable.

Yes, but if by any chance he should meet a policeman under those low trees in the Piazza de' Branca, what would happen?
A man with a hand-cart, and with something shapeless upon the hand-cart, in the dark, hurrying towards the river--such a man would excite the suspicions of a policeman.

Marzio might be stopped and asked what he was taking away.

He would answer--what would he answer in such a case?
The hand-cart would be examined and found to contain a dead priest.