Would a Slaanesh worshipper necessarily have an evil alignment in D&D? can't a character be a hedonistic Slaanesh worshipper without mutating and going insane, if so, why?
Although this is probably more adept for rpg stack, I'll answer.
Slaanesh is not only a god whose worshippers are hedonistic. They are totally insane, crazy and chaotic. This limitates your alignments to the "Chaotic" branch.
Also, they enjoy and find pleasure in pain, getting it and causing it to others, this also limitates de "Good" branch of the alignment. A Chaotic Good character couldn't enjoy causing damage or pain to others.
So, from my point of view, a Slaanesh worshipper must be Chaotic Neutral or Chaotic Evil. No other alignment in the collection fits into their primary behaviour.