Not a serial killer
Durst is a murderer who has killed at least 3 people, but I don't think that makes him a serial killer.
The first murder, of his wife, was probably an act of rage and control. The second 2 murders were his way of protecting himself from discovery. If he had managed to kill his brother, again, it would have been an act of vengeance.
I don't think this makes him a serial killer, as the murders do not fit any kind of pattern. The people he murdered or has wanted to murder are people who got in his way, for different reasons. It's not like he sought out a particular kind of person, killed them/disposed of them in the same way, or got some kind of pleasure from it. I don't think he went around thinking, 'Who can I kill next?' I think he went around thinking, 'This person has jeopardized my freedom and power, so they must die.'