Defending himself and bystandards from a credible threat. Two justifications come to mind.
1. Blake had already grappled with officers and was now reaching (as opposed to getting in) for something in his vehicle. The automatic assumption when a violent person with an arrest warrant who just fought with the police is doing here, is going for a weapon.
2. There were children in the vehicle. The police were responding to a Domestic Violence call of someone who was violating a restraining order against the woman he was accused of sexually assaulting. This is a man who cannot be left alone with children and the cops must have realized that Blake was a potential danger for those children. If he was reaching OR if he was trying to get into the car to drive away, lethal force was justified.