Thanks. Interested to see when you get a minute to dig it up.
He’s likely referring to the Rebbe saying (I’m paraphrasing heavily and might be saying it wrong)that we cannot understand Hashem’s ways and that sometimes Hashem’s goodness is enclothed in things that on their surface our cruel.
The Rebbe gave an example of a surgeon ch”v amputating a limb to save their body from a disease. An onlooker might on the surface see the surgeon as vicious, cutting off someone’s limb, if they don’t understand that it’s a kindness to save the person’s life.
The Rebbe wasn’t comparing the holocaust to a cancer at all, he was just suggesting that we cannot understand Hashem’s kindness and used an analogy that can be misinterpreted to mean that.