The Eldar society was destroyed by becoming too hedonistic. Their partying accidentally creating the god of sex, drugs, and rock’n’roll. It likes eating Eldar souls.

As a result, the majority of Eldar become “craftworld eldar” who are very into strictly controlling their emotions and discipline. They keep their souls is little gem things to keep them safe.

The Dark Eldar decided to go the opposite direction. Dark Eldar capture other lifeforms to torture them, as a way of keeping the dark god they’d created from eating their own souls.