Why do some crucifixes have skull and crossbones on them?

Answer 1

The skull and crossbones, placed at the bottom of a crucifix (beneath Jesus' feet) represents the legend that Christ, the New Adam, who conquered sin and death, was crucified on the gravesite of the first man, Adam, who brought sin and death into the world through his disobedience to God. The skull is a symbol of Adam's grave - antique prints of the Crucifixion often show a skull at the foot of the Cross. Most important of all, the skull and bones are placed beneath Jesus' feet, to show that He triumphed over death and sin on our behalf..

Also, Christ was crucified at Golgotha which means the "Place of the Skull"

(Taken from related link below )

Answer 2

There is an old tradition that Jesus' cross was erected above the grave of Adam and that some of His blood dripped down through a crack in the rock onto Adam's bones.

There is a large painting in the Church of the Holy Sepulchre in Jerusalem (behind the Stone of Unction) depicting this tradition fairly graphically.

There is no Biblical basis for this tradition, which probably grew out of St Paul's assertion in 1 Corinthians 15 that "in Adam all die, in Christ, all are made alive".

Also Christ was crucified at Golgotha or the place of the skull.

Answer 3

There may be different reasons, but the skull and crossbones was one symbol used by the SS. If the skull is turned slightly to one side and resting on top of the bones instead of floating above them, that is probably it.