Self-defense, in the UK, is essentially illegal
It would seem that these guys way over-stepped what could be deemed "self-defense" - they meted out punishment to the gang member they caught.
Under Swiss law, they would still be given a lenient sentence. Charges might even be dropped. The legal principle applicable is "a state of extreme excitement due to the circumstances".
In the UK? A prison sentence. Although the "victim" was a serious repeat offender.
http://www.theguardian.com/uk/2009/dec/14/jail-brothers-burglar-cricket-bat