I just spent most of today trying to catch up on this case. First off, my heart goes out to Tim's family and their great loss. While getting into the sleuthing side, I don't want to discount their incredible loss.
Now on to a few comments about the accused. I know there were discussions here about drug use and gaming, etc. I am fine with occasionally playing video games and smoking pot, and I know highly productive individuals that do both. That is not the point of my comments here.
The motivation for this horrible crime is a mystery. My impression that DM has some kind of personality disorder. Antisocial? Sociopath? etc. a label is not really important. Based on family history, it is highly likely that he is an addict, alcohol and/or drugs. It seems to me that he is in his late 20's and still functioning as a teenager. This is very common among addicts. They basically stop maturing at the point where addiction takes over. This is often in the mid-teenage years.
He is also a spoiled brat. (sorry, I just don't have time to go into a better description). It has been stated that his father was in substance abuse recovery. If this is true, it is highly likely that DM was overly "spoiled" by his father to over-compensate for his addictions. I hope this does not offend anybody, but it is a typical dynamic of families with addictions.
Edited to add that some of my comments are reactions to DM's profiles on various social networking and gaming websites. Posting such profiles and pictures for shock value is common among teens, not so common once somebody has reached the mid to late 20's.