The backgrounds aren't unknown. Some of those who have been charged still have their Facebook profiles online (although I expect they'll delete them) and people are circulating them on social media. The ones I've seen appear to be very "normal" - family photos, animals, jobs, etc. If anything I find it more frightening that these are probably "normal" young adults. For all I know, their grandparents might be on GN. I really don't want this kind of behaviour or attitude to be the new normal!
Somebody posted that Croydon is full of gangs, as though that excused what happened. I don't know Croydon at all, so I checked the crime statistics. Admittedly it's higher than average, but it's not unique. There are plenty of places in the UK which have higher crime rates, so maybe this kind of thing is going to be the new normal.
There is no excuse, whatever their background.
I accept that Brexit didn't cause the attack, although I still feel that the hysteria surrounding and hatred directed towards asylum seekers has created a toxic atmosphere.
I expect you remember the vile sentiments expressed by Katie Hopkins:
www.dezeen.com/2017/02/14/dezeen-launches-unofficial-brexit-passport-design-competition-1000-pound-prize/
Even David Cameron called them a "swarm".
They weren't the only ones. Looking at Twitter yesterday, there are plenty who still think like that.