It's possible and one of the simplest theories that Donald did indeed kill Brittney before taking himself out, but that logic doesn't work when we now know that there is more than one other surviving female victim as well as Derek and probably more male victims, too. Why would Donald single Brittney out and not any one of the others that could also testify against him? That's what makes me lean towards his "suicide" could very well be staged. November will be a year that Fox10 reported the suicide investigation was being reopened but I don't think there's been any public updates since then.
I think it's possible that DH thought he could manipulate the testimony of the other/s but Brittney was not having it. She wanted to tell the truth, and was encouraging the other/s to do the same.