It seems to me the shooter wasn’t defending himself after he drew his weapon. He was acting in retaliation.
We weren't there, it looks to me like the guy was forcibly thrown to the ground, looked up and the perp was standing there within feet, surely close enough to be re attacked in a second and killed. At that point he pulled his gun and shot the perp not knowing what the violent perp would do next. These events occur in seconds and the person attacked has one second to respond and perhaps save his life.