Suppose one of your family members is murdered, God forbid, and you know who the killer is?

[Leo]

you love the victim very much, say a brother or sister. You witnessed the crime, or have irrefutable evidence of the crime against the killer.

But the murderer is aquitted on a technicality or because of botched police work, or court procedures, or even an insanity plead, or good defense attorney, etc..

What would you do? in other words when is it alright to take matters into your own hands?