You mistake justice with a legal trial. This was rough justice and we can all recognized it. The police officer should be tried since he acted beyond his authority. But still that does not mean he did not enact true justice.
There was a confession and then the ultimate penalty.
Individuals delegate reponsiblity for justice to specialized elements such as police and courts. But still justice is the responsibilty of the people. That is why we cheer this action even if it was illegal.
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.