It's not crazy at all.
They may have started off defending themselves but they ended up chasing them down the street seeking vengence.
I don't know how badly one was beaten but it doesn't sound good.
If incidents like this go unpunshished we'd have lynch mob law with people taking justice into their own hands.
There have been a number of high profile cases - Tony Martin is the most famous who shot an unarmed burglar in the back with an illegal shotgun.
He was convicted of murder but it was reduced to manslaughter on appeal when it came out he had mental health issues.
Not as bad as your case but shows you the seriousness with which such cases are taken.
This is a closer case
http://www.mailonsunday.co.uk/news/article-106 6777/Brazilian-student-faces-GBH-charge-defend ing-home-serial-burglar.html
The tabloid love this sort of stuff but the law takes a very different view - this is Britain not the wild west