I wouldn't call it murder because he was coming at them with a weapon, but I don't feel that police should always resort to lethal means, this being one of such occasions. Do they even bother with tasers and pepper spray anymore?
Tasers don't always work. Rubber bullets don't always work. Bullets don't always work either, but they work more reliably than the former two options. And if you have a psycho with a knife coming at you, I will bet fucking anything you will go with the option that has the best chance of saving your life every time.
And anyone who says otherwise is either a liar or a fool who will change their mind immediately should they ever find themself in that situation.