I know that someone was beheaded. It is clear that this is an horrible and cruel act, that nobody and nobody's family should experience. What information does it add to watch the video? You can convey the relevant information in text.
No, you can't. The fact you think so is the entire problem.
I think so, too, and I don't think it's a problem. Rather than just telling people they're talking out of their ass, why don't you explain what value is gained by watching it? Obviously there's no factual information in the video that can't be expressed in a few sentences of text, so the only think I can suppose is that you're of the opinion that the greater emotional impact of seeing it has value.
What, precisely, is that value? For me, personally, I can't imagine what it would be. I don't think anything could make me more strongly opposed to the act of beheading an innocent journalist. Seeing it would make that opposition more visceral -- perhaps in an almost literal sense -- but it wouldn't increase my opposition. It wouldn't lower my opinion of the terrorists, either, since it's not possible to hold a lower opinion of them than I do.
So what is the value of seeing it?