When you go to "beating someone to death because they're bad and they've hurt you and yours" rather than "beating them just enough to stop them hurting someone", you've made the transition from hero to murderer.
Yes, they did wrong. Yes, they deserve to be punished for it. Yes, it probably deserves to hurt and to last for a long time.
But there's a justice system for that, and your fists aren't part of it.
You say you'd take your chances with a jury - well, the jury can only deliver a verdict on the evidence they're presented, and if that evidence is "he unnecessarily and gladly beat someone to death, and did so on camera", then you goin' down, son. The prosecution will make sure of that, painting the deceased as a troubled, tragic figure, and you as a violent ex-marine. Not to mention that you've got a family who presumably quite like having you around, earning money and being there and generally not doing the whole "in federal prison" thing. Why roll the dice with that?
There's no place in a civilised society for vigilantism.