Nobodys going to mourn for him, and the world is a better place without him, but its not justice.
Justice is when you're arrested, put on trial, found guilty and then sentenced according to the laws passed by Parliament. Not when some random decides to stab you in the shower. And lets face it, that random is unlikely to have been a standup member of society, given he was in the same prison as someone was awful as Watkins. Lets not outsource 'justice' to such people.
And if we decide that, yes, Watkins deserved to die, then we have mechanisms to introduce that.