I agree, it appears there was something about 'eliminating' people that was psychologically gratifying to DM, tho it doesn't seem he was sadistic like most serial killers. Instead, I think he was cultivating a kind of ruthlessness that is admired in young men's popular culture.
The tattoos on his wrists were ambition and discipline. My problem with the phrase 'thrill kill' is it implies fun and games, and a lack of discipline.
I think he was more into modelling the culture of some video games than anyone has discussed. There, you run 'missions' which require discipline, cunning and ruthlessness to succeed. There's also shows like Breaking Bad, he cited in a letter to CN, which many young viewers completely missed the moral point of. They just enjoyed seeing Walter get more and more ruthless.