Dear Two,
The police were called for a domestic incident. Their only duty is to settle the incident. There is no obligation on them to walk the beat on a call. None of those reasons automatically require the officers to remain on the scene.
None of the factors you listed are clear cut:
A: According to Raysean White, he was not even resisting. At the time he was shot, the tussle had ended.
B: To arrest him for the warrant, they would have had to identify him first. And then attempt to place him there arrest and read him his rights.
C: There is no solid corroboration that the knife was a factor. I doubt the police ever noticed it.
D: It was his kids in his car with his knife. Why is this a problem? Every father that takes his kids fishing does this.