He shot them dead. It’s a figure of speech, meaning the shot was fatal. So the headline is fine. He shot dead a cat. He shot dead an old lady. He shot dead a black kid.
The headline is technically grammatically correct but ambiguous. "...shot and killed unarmed black man" would have been better. If you absolutely need to stick to word/character count, "shot unarmed black man dead" would be less ambiguous and more in keeping with how people actually use "shot dead". I've watched a lot of westerns and I can think of quite a few where someone says "I shot him dead" but not one where someone says "I shot dead him".
Comma would be weird; there's no pause, for example: "he shot dead a heard of cattle". It all flows as one line.
If you want to fix it, just ad an "an":
"London Cop Who Shot Dead An Unarmed Black Man Charged With Murder". Which is typically how the saying is used. If you want a comma, I'd add it after "man".
Figures of speech turn up in all writing, and especially in headlines. They’re useful to convey more meaning than is normally possible in few words because they rely on assumed context. Because major goals of headlines include information packing and brevity, idioms, turns of phrase, and figures of speech are common.
Yes and no, technically without "dead" and leaving the rest of the sentence unchanged, it could imply the cop has a separate murder charge. A better headline is "London Cop Charged With Murder After Shooting And Killing Unarmed Black Man."
That doesn’t rule out the logical possibility of the murder charge being for a different incident. But you could eliminate it with “London Cop Charged With Murder For Shooting Unarmed Black Man”.
This whole article has a few confusing points, and the article never clarifies them. So the vehicle is confirmed to be involved with a shooting the day before. The vehicle does not belong to the rapper, but was driving it. How did the rapper acquire the vehicle? Was he actually involved in the shooting the day before or not? Did someone he knows give it to him? Lots of questions, bad article.