The 'court of public opinion' has its own lingua franca, and throughout the ages has coined its own words and phrases to describe those it regards with contempt. "Ratbag" for example, was used in Victorian times and still sometimes used today. Some of these epithets resurface as well - like "villain", "yokel". "Scum" and "Scumbag" are a continuation of this tradition.
And if anyone thinks it's only the 'lower orders' - or 'lowlife' - who use this terminology, they're mistaken. "Scumbag" is informal and derogatory and it's used to describe a despicable person. And I've definitely heard well-bred individuals use the terminology when sufficiently incensed.
This 'tradition' is not going to change. I think Rafichagran associates it with the 'mob' mentality. And that does exist and it's ugly - but I don't think any gransnetter would be in favour of joining a baying mob intent on lynching etc.
I think the bias against Zouma is really more to do with him being an over-paid footballer whose position has 'gone to his head', as it seems to do with some of them. Others, like Marcus Rashford, use their fame and fortune for a worthy cause - as did David Beckham, at one time (maybe still does, I don't keep up with the Beckhams' lifestyle).
Like it or not, footballers are seen by some as role models. Zouma has set the most appalling example - there have been copy-cat incidents of people abusing their cats and videoing it for social media. He could've killed the cat with his kick (his drop-kick). I don't want to see him lynched by a mob, nor beaten-up, but I won't hesitate to call him a "scumbag" which the informal dictionary describes as someone who is "a contemptible or objectionable person". That's exactly what he is.
