I don't know the specifics of this case, if any of the robbers had guns, but the news states that the deceased burglar was tried for murder a number of years ago, and there doesn't seem to be a conviction for that offense, for whatever reason.
As stated, Salford is it's own city, seperate from Manchester, but very divided, parts are very run down, tower blocks, and graffiti, and other parts are super new, the Keys, Lowry centre, the new media city. Manchester is pretty cosmopolitan, and crime free comared to others, and Trafford, just seems to be a shopping centre, and an industrial estate? pretty tired in most areas.
I think the difference between using a knife in a robbery, as opposed to street crime, and using a gun, is the reality of the end result. If you are robbing a corner shop for a few hundred quid, you are probably a junkie, looking for easy money, where a gun would be used in a bank, for higher amounts, and more planned. You know that 50/50 if you use a knife, someone might live, but a gun is pretty final.
I've had a gun pointed in my face in a robbery. It is an experience that never leaves you. Unfortunately, the machine they were robbing had no money in it, and they managed to angle grind their fingers off in the process. So police arrived, at the time they were making their getaway, and nearly ran me over, had them in prison before the day ended, as they literally 'left their fingerprints at the scene'...