I'm not sure if it stacks with Death Strike (the skill) or Hammer Mastery's % chance of doing a deathblow or not, but yeah, it's basically useless. All animals (bees, snakes, spiders, etc) have 100% spiritual resistance, and later in the game, you won't encounter anything with less than a 50 in all resists, so it'll never happen. If you have a good weapon that also has deathblow, fine -- enjoy the thunderous shaking as what you're smacking dies suddenly -- but don't count on it happening very often.
