The Darkness Easter Egg - Full Metal Darkling

If you look closely, after summoning the Gunner Darkling, you'll notice he has a military helmet on. Stand directly in front of him, after he stops moving (likely will need to be sure no enemies are around or he'll be slaughtering them).

The words "Born to Kill" are written in white which is a direct reference to Private Joker's writing on his helmet in Stanley Kubrick's 1987 Vietnam war film "Full Metal Jacket".