As you mention, the lamb is typically depicted as a symbol of peace and gentleness; in fact, the lamb resounds with Biblical symbols of peace. In “Lamb to the Slaughter”; however, the frozen leg of lamb symbolizes much more dreadful and dangerous—it symbolizes violence, death and revenge.   I also believe that it is a perfect example of dramatic irony, as well, because the audience is aware of what is going on as the officers eat the lamb.  The characters are not aware, however, that they are eating the murder weapon.