I'm not sure why everybody celebrates these violent events.
First of all, the people who murder paedophiles in prison are not saints. The man who murdered Richard Huckle (one of the worst paedophiles on record) is a sex offender who is so dangerous that it is not permissible to leave him alone with a female prison officer under any circumstances.
Second point is that this is a symptom of prisons not being secure enough. If Ian Watkins has been murdered, who else could potentially be harmed?