I was doing some reading on thrill killing and ran across criminal psychologist/profiler site. It got me thinking about the thrill killing angle that was talked about a bit early on. It somewhat answers the questions for me regarding motive (thrill killers dont need one, though it's been stated DE's motive was to silence Nicole) and why they were so sloppy with evidence.
Some notes on thrill killers according to the profile:
...Therefore, the thrill killer will actually aim to befriend or understand their victims better to earn or receive emotional acceptance. This type of acceptance can take place in many forms but is typically experienced through respect and appreciation.
... The alarming characteristic is that thrill killers are not considered to be insane. Thrill killers process data differently from a psychological perspective and experience a euphoric disposition from planning and plotting heinous and sadistic scenarios that always seem to astound society.
... Thrill killing is typically brewed upon hate and rage and yet there doesnt have to be a motive associated to the killing (Thompson, 2003)
...Thrill killers are time bombs, that want their victims to need them, to be socially attached to them, and there doesnt have to be a motive present to fuel their killing, just an opportunity. It is rare for a thrill killer to hide or cover their killing or the evidentiary aspects associated to crime commission because they dont believe it is wrong.
http://ericahutton.blogspot.com/2011/02/examination-of-thrill-killing.html