If the person who stopped him had just restrained him (with whatever physical force was necessary) I don't think that would be any likelihood of the 'hero' getting prosecuted.
But if he he continued to physically assault him past that stage and, in this case, resulted in the death of the bad guy. Then a prosecution should be considered.
Maybe the have-a-go-hero is a really nice guy but he could also be a bigger piece of scum that the deceased.
The just giving page seems to show one person giving £1,000 and other friends make up about £350.