"But more than that, it might make more impact on someone who's not yet committed a similar crime but is in danger of doing so, ie young lad, does a lot of posturing with his gang, carries a knife, brags about cutting someone etc etc. Hopefully if they ever got to see something like this they might think twice"
Well, like you say, hopefully. What if it does the opposite.
You know that when suicides are reported, there is always a spike in suicide rates, don;t you? Or that gun crime and knife crime accelorated at a rate which corrolates to the medias obsession to it?
Maybe the media should start talking to criminologists rather than justifying the risks they themselves pose to public saftey in the quest for viewers.
But why stop at CCTV - why not show kids all kinds of torture and atrocity captured on video if, as you suggest, it will get the message across that murderers are 'losers' and idiots. As if someone goes out and thinks, 'tonight I will/will not be a loser'. Life is much more complicated.
Have you no idea of the kudos within gangs? As if lecturing them or showing them fuzzy images of kids being knifed will change the life history that leads some people down the road of violence.