I don't have any expierience with DE in RT, but I'd like to point that - just as in cases of SoB and Marines - a Dark Eldar on board of RT vessel isn't going to be yet another dark eldar #121242.
A sociopath and total psycho - yes ofcourse, but he just have to have something special. I.e. politics: why not have a DE forced to serve (and "serve" is *the* word) on board human vessel as an wicked entertaiment for some Kabal boss who is looking at him almost constantly?
As to the daily killing - resolution arena, disciplinarium, brig and list go on and on.
Apart from being psychos DE are described as more inteligent than humans and they have technology and skill far beyond those of Imperium. Look at HBOs Dexter - there can be a sociopathic serial-murderer in a normal sociaty without causing any big turmoil.