The picture is itself evidence explaining why I believe the kitchen knife may have left the imprint, and thus, why there is no need to assume that the handle was covered in blood.
Nobody is required to "believe" things based on a picture, but I note as a matter of facts that nobody presented arguments to disprove it.
I will attempt to explain how an image is produced by an object coated with a visible liquid. I don't expect you to understand, but there may be some person reading this who may want additional information.
I will start with the concept of
letterpress printing. This is the process where individual raised letters, or blocks of raised letters, are placed on a platen, inked, and a sheet of blank paper placed in contact with the inked letter assembly. A second, flat platen is then pressed against the back side of the paper that is resting on the inked platen (which has the raised letters).
The images of the raised letters are transferred to the paper.
{This, of course, was the invention attributed to Gutenberg. See: http://en.wikipedia.org/wiki/Letterpress_printing}
Now consider a knife that has blood on it - and the knife must be soaked in blood because it has seconds ago been used to cut or stab someone's throat, and has severed one or more large blood vessels. There was a spurting of blood when this happened, because blood circulates in the body under pressure somewhat greater than atmospheric pressure.
The knife then comes into contact with a flat sheet on a bed. Just as in a letterpress printing machine, there is a transfer of the liquid - blood - on the points of contact from the knife to the sheet. Transfer from any recess on the knife not contacting the sheet is unlikely. And transfer from the air around the knife to the sheet is impossible.
Therefore, the image of a solidly-filled in part adjacent to the back of the blade must be the handle of the knife. The curved image is the curved cutting edge of the knife. There is some indication that the knife was wobbled (moved slightly) on the bed sheet.
The flat blade of a large knife would not reasonably create the image of the handle on the sheet.
The knife which made the image in blood on the sheet is thus considerably shorter than the large kitchen knife suggested by Machiavelli, and the depth (curved edge to straight edge) is also much less.