Let the boy try along this bayonet-blade How cold steel is, and keen with hunger of blood; (1-2)
These lines sound like they are spoken by a drill sergeant or an instructor, somebody who wants the "boy" to prepare himself for war. The lines also paint a picture of warfare that is devoid of emotion and life—the steel is "cold" and desires "blood."
Blue with all malice, like a madman's flash; And thinly drawn with famishing for flesh. (3-4)
The simile comparing the bayonet-blade to a "madman's flash" makes war seem pretty crazy. Moreover, war is so bad it doesn't just kill people, it eats them. Notice that the bayonet is "famishing for flesh." Gross, right?
Lend him to stroke these blind, blunt bullet-heads Which long to muzzle in the hearts of lads. (5-6)
The word "stroke" is kind of erotic. As awkward as it sounds, the speaker wants the "boy" to feel affectionately about his bullets, to treat them like some kind of pet or lover or something. It sounds like he's encouraging the boy to take some kind of perverse pleasure in weapons and warfare, don't you think?
Or give him cartridges of fine zinc teeth, Sharp with the sharpness of grief and death. (7-8)
War is all about killing, death, dying, and the like. The rhyme on "teeth" and "death" speaks to this point. A normal body part, which has a lot of uses (chewing, smiling) is somehow made to represent a killing implement.