When you are in the "Otherworld" (you will go there after you die the first and second time), the Germans soldiers almost always get back up after you kill them. The only way to make sure they remain dead is to devour their hearts.

The following is an easy way to keep the zombies dead when you die. Kill them with the demon arm, and throw them off the ledge. Note: This only works outside. If you throw them off the ledge inside a building, they will not die. Instead, they will follow you up to the level you are on. You can also devour their hearts to keep them dead.