Let the boy try along this bayonet-blade How cold steel is, and keen with hunger of blood; (1-2)
The bayonet-blade sounds suspiciously like a vampire here. Hungry for "blood"? Yikes. This is a creative way of describing the way in which war is plagued by death. Weapons of war drain soldiers of their "blood," their life-force. It is not just those who actually die that lose their blood, however, but also those who fight and survive.
Blue with all malice, like a madman's flash; And thinly drawn with famishing for flesh. (3-4)
The cannibalistic qualities of the bayonet are rearing their ugly heads. The blade is "thinly drawn" (skinny) because it is starving, "famishing," for flesh. Weapons have a life of their own, and all they want to do is kill and eat people. Lovely sentiment, Mr. Owen.
Lend him to stroke these blind, blunt bullet-heads Which long to muzzle in the hearts of lads. (5-6)
The poem likes to blame weapons, but these lines throw a wrench into that effort. The bullets are "blind," which means somebody has to guide them to their targets. They are "sharp," sure, but it seems like those who are doing the guiding are really responsible for the killing.
Or give him cartridges of fine zinc teeth, Sharp with the sharpness of grief and death. (7-8)
The rhyme on "teeth" and "death" emphasizes the fact that bullets are made for killing, for inflicting death. The cartridge is imagined as a face or mouth with sharp teeth, and it kind of seems like a carnivorous monster, just like the bayonet-blade of the first stanza.