Sure from our perspective. But on a crowded subway when the large deranged man is cornering the girls it's not as easy. And pointing out his derangement doesn't help your point, it actually hurts it. Reasoning with someone who is insane is almost by definition far less likely to succeed.
A choke hold that (likely accidentally) results in someone's death in that situation could very easily be the most reasonable course of action to protect the health and possibly lives of the girls he was targeting. By no means should anyone enjoy doing it, watching it, or that it happened. But your position is very easy to hold when far removed in both distance and time.
And, for the record, that death wouldn't be murder. Or at least that's the entire argument. That would require the death to be intentional and without legal justification. A choke hold in defense of others meets neither standard in this situation, imo.