Difficult one.
It feels like it's easy to blame football, but the problems are deep-rooted societal issues. It's lad culture to the extreme - hundreds of thousands of (mostly) men choosing to use football as an outlet/release, the same way a week in Ibiza going mental is a release.
These are people let down by the lack of opportunities in the UK for working class, made bitter by Brexit politics, looking for an excuse to get wrecked, show off to their mates and get 2 minutes of fame by doing something ridiculous like firing a flare out of their ****************. I was in London on Sunday and the amount of coke being done openly on the streets from early in the morning was insane. It's the same at football matches now, coked-up lads looking to go mad but not necessarily for a fight like the earlier hooligans from the 70s and 80s.
Sunday was a a perfect storm of providing the excuse for hundreds of thousands to congregate on one area (Leicester Square and Wembley), a late kick off time, the build up from a month of hype and woefully inadequate policing and security.
Fixing the security would no doubt have mitigated some of the problems but doesn't get rid of the wider issues. People will continue to act like that. We see it every weekend on the streets of most UK cities, just not at such extreme scale. Not sure how you fix it - countries like Spain and Italy have far worse unemployment than UK for the young, yet you don't see the lad culture there.