I believe vigilantism should only be used for the worst crimes. Don't go ham on some guy who shoplifted, But the killing of murderers, Drug dealers, Rapists, Pedophiles and terrorists seem fine to me. As long as you have enough evidence to know that the criminal actually committed the crime, Go for it. There are several cases where criminals get away or have minimum time, Or even worse, Get millions of dollars because of their actions, Such as Omar Khadr's, Where he killed a US Medic, And then got compensated $10. 5M for being imprisoned in the States as it "violated his human rights". Justice should be served, Whether through the judiciary system, Or when that fails us, Vigilantism.
In this country (U.S.A.) you are considered innocent until proven guilty in a court of law. Without this principle, many an innocent person would be put in jail or killed.
What we could do is work toward streamlining the process. After all, we also have the right to a speedy trial so after the defendant has time to talk to a lawyer, they could be in court, defending their innocence that very day. If it was a crime punishable by death, and there is sufficient evidence to prove guilt such as video tape, they could go right from court to the means of their execution.