From the article I just read, I don't think anyone stood a chance of stopping him. The poor boy was down from the first assault and he planned it, he separated him and he was prepared for what he was going to do. This wasn't a rage turned murder, I don't think he started shouting and threatening, he just just did it from the reports.
As much as I would protect any child if I could, and have flown into the dark street when I heard screams despite being short and fat and unfit - my kids need a mother and I cannot subdue a madman coming at me with a knife. If the police couldn't get him with capscium foam and had to shoot, the people there couldn't do anything without potentially dying either. And I think poor Luke was murdered very quickly. As callous as filming may seem, it is the perhaps the only safe and useful thing to do once the authorities have been called. If he had survived that would be evidence for the courts.
I am so upset by this. I wish there was a way to protect people from the madness amongst us.