Not saying the holocaust was a myth... I'm saying making soap out of the bodies
was. It was a misunderstanding among Jews and translation of the stamping on the soap itself:
https://youtu.be/_hLWyf3m8k4
https://en.wikipedia.org/wiki/Soap_made_from_human_corpses#World_War_II
Also, think about how cost prohibitive it would have been to slaughter and render the bodies of the victims, for just the fat, and nothing else. If it happened, it would have been at a cost and for just pure spite.
*edit*
If there was an eye witness or an account of an eye witness making/rendering, then I will repent of my stance. But from what research I've done, it was a rumor that circulated in the concentration camps, potentially an experiment in a lab once, but not much more than that.