The Totenkopf was first used as an insigna in Prussian army in 17th century, after the 30years war. It remains as it untill the Third Reich when SS "stole" it from regular cavalry regiment.
"Regular cavalry regiment" who were in fact light armored tank, hated SS troops for that.
If you have read Panzer Lieutenant by August Von Kageneck, there is a line about that. So its kinda like the swastika, it was used before Mr Hitler was born.