First of all, "true" justice is reserved to God, and God alone.
Secondly, are you honestly willing to trust lives to the word of a cop and his friends? Yes, in a way I'm happy this scumbag got what he deserved (as far as we know), but cops should NEVER decide guilt.
Is a trial equivalent to justice? Of course not, but it's a heck of a lot better than trusting a thug with a badge.