The reason the intended use of a man-made object is so important to this discussion is that we have a tendency to redesign things until they perform optimally for that purpose, and not so efficiently at random purpose X. Objects that are not man made rarely have the same qualities required for causing deathness as their fabricated equivalents.
Hence a sub-machine gun is great for cutting down a crowd of rampaging monsters, whereas a kitchen knife isn't. A pistol is great for conventiently portioned stopping power, but a stone is just going to take someone's eye out*.
* 'parents' joke