I think the murder of Lucas and Chynna was likely what is called a 'thrill kill'.
There have been pointless murders by pairs of very close young men where, in retrospect, one appears to have been a psychopath and the other a devoted admirer, who egg each other on where they might not kill on their own.. They turn to crime because they like to imagine they can live outside social rules, and because it makes them feel powerful. The famous case was Leopold and Loeb: They also were 18 and 19 years old. They kidnapped and murdered a boy just to prove their perceived superiority by committing the perfect crime. Eric Harris and Dylan Krebold, the Columbine shooters. were 17 and 18, Harris has been labelled a psychopath who was contemptuous of everyone else and wanted to prove his superiority by blowing up the whole school (but failed). Dellen Millard and Mark Smich appear to have had a similar dynamic, I think, where Millard was supremely arrogant and Smich was his admiring sidekick.
Some articles have described KM and BS as spree killers. I’m not qualified to give an opinion about that but I watched an interesting show on the CI channel recently much of which was devoted to spree killers. A leading criminologist, a forensic psychologist who has worked within the prison system for many years, together with several other forensic psychologists provided some interesting insights into this particular class of killer. (Spree killers are defined as killing 2 or more people without a cooling-off period, or over a short period of time, in different locations.) Here are some of their comments.
Spree killers usually commit random assaults against people they don’t know.
They want to get revenge from either individuals or society in general.
Many of them often perceive themselves to be victims.
Spree killing may start as perceived justified killing of someone who has wronged them, or a number of people who have wronged them, and it can escalate into random persons.
The motive is typically revenge to redress the perceived injustice, and once they’ve exacted that revenge, they don’t tend to keep seeking out other victims.
They have issues throughout their lives but don’t deal with them like normal people do.
They tend to take things personally which they internalise. Over time, they’re like a volcano that is bubbling beneath the surface.
Their behaviour becomes a bit more erratic or random over a period of time until the day they wake up and decide to wreak carnage on others.
One of the characteristics of spree killers is an aim to commit suicide by either putting a bullet in their own head or “suicide by cops” where the police will kill them.
I also read an article that said that they will also kill others who will meet their personal needs at the time. In other words, they will kill for money, drugs, or other items if need be.
(The FBI doesn’t include spree killers as a separate category any longer as distinct from serial killers. They felt the cooling-off period which had previously defined spree killers created arbitrary guidelines which didn’t provide any real benefit for use by law enforcement, however psychologists still find the term useful).