As title - do you think the dark Eldar's evil personality is down to nature or nurture? As surely an upbringing in Commoragh would inevitably produce the sadistic killers that the Dark Eldar are, however, say for some reason that a dark Eldar child or what have you was taken from Commoragh and brought up by craftworld Eldar, do you think they would effectively become a Craftworlder instead (albeit with a slightly sadistic nature depending on when they were taken from their dark kin).
Anyway, over to you :)
Firstly, I want to say I don't think the Dark Eldar are evil so much as selfish and not fooling themselves, unlike the Imperium.
Well, unlike the Craftworlders who managed to escape the creation of Slaanesh and now rely on Spirit Stones to stop Slaanesh from enjoying their souls, the Dark Kin had a different form of injury. Whereas those in the material universe simply died, eaten by Slaanesh, or killed by the resulting Warp energy, those within the Webway and those descended from them have this curse to be constantly drained of their souls.
As such, a child of the Dark Eldar would be forced, by their very need for survival, by the inborn curse of Slaanesh, to find someone to suffer on their behalf and rejuvenate them.
Even if you took a child from the Dark Eldar and gave him to the Craftworlders, he would simply die after a while because his soul is slowly being drained. The Craftworlders would be completely unable to help him, as the Dark Eldar have let their psychic abilities die to avoid Slaanesh, whereas nearly ALL Eldar technology is based upon the innate psychic ability of the Eldar.
Moved to Fluff and Stories since it's not discussing gameplay in any way.
Some are born evil, others die very quickly.