If the victim was an imminent threat, then police training would have required at least a dozen other police officers to behave differently. Instead, we know that only one officer fired his gun. How many had their weapons drawn? If the victim was an imminent threat, every officer at the front door would have been required to draw their weapon or act to secure the scene. But that's not what happened. One, standing next to the subject officer, is said to have her arms crossed, such was the imminent threat. (I can't make that out on the video, so please correct me if those reports are incorrect.)
At no time did the victim "lunge" at the officers, and I don't know how many of you have ever descended stairs, but to do it quickly and then be able to attack someone who is five to ten feet away from the stairs, while you only have one hand to balance with, takes some doing. A superior (higher up) position is an advantage if you have a gun, or if your opponent has to meet you half-way up, but becomes isolating if you have a three-inch blade: you can't hold it low enough to maintain the level of threat.