Hmm, I think all those motives have in common that DM wanted something but didn't get it or couldn't have it so he got rid of the person keeping him from what he wanted. His tattoos would explain that mentality.
With regards to the LB murder, there has to be a reason why she called him eight times the night she died. She was either in love with him, needed something from him or blackmailing him? All is possible, but he probably did not "need" to kill her it could be over something completely unexpected like she had a drug habit, she couldn't afford it and he no longer wanted to pay for it in return for favours. Plus, his girlfriend probably didn't like it so he got rid of her.
With regards to his fathers murder his dad was doing stuff with the company that was not going to be profitable and he did not agree with it. So, he killed him to get him out of the way so he could do as he liked with the company.
With TB's murder, motive is pretty clear he wanted to sell the trucks parts and make some money of it. TB is in the way of what they want so they kill him (not necessarily physically in the way, just his presence).
I think what would be interesting to find out would be MS's motive because he's largely stayed out of the limelight because all the focus has been on DM. I wonder if he was being paid by DM for this or something? He probably would have gotten a cut from selling the TB's truck. Also, he clearly had an interest in murder and gore. Maybe, he was recruited to help with LB's murder in an innocuous way (clean it up) but then DM blackmailed him and then the second time he couldn't get out of helping him?
So many maybe's. IMO