Societal ills, such as knife crime, are inherently undesirable within a society. In all spheres of life we try to reduce or eradicate them. By showing the stabbing, East Enders raises awareness of the issue within circles who may otherwise be quite content to ignore it. Raising awareness is essential to tackle ills such as knife crime within society.
Image you have a country in which there is a severely oppressed minority. Most people don't care about this because they lead their daily lives without coming into contact with this minority. They then go home and watch soaps that do not include this minority, or simply portray them as living harmoniously within society. The problem never gets solved because there is a lack of awareness. Now imagine that soaps start showing the effects of oppressing a minority, be it people from that minority becoming involved in crime or themselves being victimised. This raises awareness and puts this particular ill on the public radar. The same can be applied to knife crime.
Therefore, to me at least, it seems pretty clear that not only should East Enders be allowed to air such scenes, but by airing such scenes they are actually beneficial to society, albeit in only a small way