I don't think that DM killed LB because he got a different girlfriend... otherwise, why not just totally ignore her, block her texts and calls, block her from FB.. stop communicating with her.. stop having sex with her (MSM made it seem, imho, that the two were still having casual sex well past the time when DM and CN were an item. I also find, through looking around me over time, that it seems that some men kind of like when two women are squabbling over him.. that's just a personal POV. It seems to me that he became agitated with LB when she was pestering him - wanting a place to stay, wanting him to supply her with drugs, wanting to engage in sex with him when he'd already said enough, etc. It seems that LB didn't really have a money issue since she had dropped off a few thousand dollars at her parents' home just before she 'disappeared', so I doubt if she had been pestering DM for money.. however, I wonder if LB had been making any kind of blackmail type of threats to DM, perhaps regarding things he'd said to her, maybe photos secretly taken, and sexual encounters, - threatening to expose things he didn't want exposed, to CN. He also had developed his idea of incinerating humans to make extra cash, and was probably dying to try out his experiment, to see if it could potentially become a viable source of income for him in future.
Then his dad was being pesky, perhaps moreso since WM had developed new stamina because of his budding romance.. and time was becoming of the essence for WM to get a move on with that MRO operation.. DM was likely used to living a life by his own rules and time, and probably found WM to be getting more on his nerves. He had to go too.
I think JDG is correct, it was only a matter of time before his mom would have encountered a similar fate. He'd inherit whatever earthly possessions she had, plus save himself a few thousand bucks every month in not having to keep paying her. No more silly questions to have to answer to, nobody 'helping' him to run things. She was his biggest fan, so it's hard to imagine, but once he got a taste of how easy it was to get rid of people with no apparent repercussions, I can imagine this was only the beginning.
All just my interpretations..