Do you think the crime of murder is unjust?
How about murdering a terrorist?
Since you call it murder I am going to assume you mean extra-judicial killing. I.e. by an angry mob or by a victim/relative.
Firstly, revenge is generally not something we advocate for, and the legal system is not there to provide revenge for victims. It is there to protect the public and provide justice.
Extra-judicial killing of a terrorist before or after he commits a crime is not just.
For one, extra-judicial killings would not meet the strict requirements for proving guilt. In other words, an angry mob might well kill somebody innocent, even if they 100% believe he is guilty.
Extra-judicial killing of the terrorist/molester/etc after they have committed their crime, well by definition it is not just. No act of vigilantism is just. It implies that people can take the law into their own hands and act as judge, jury and executioner. It denies all parties the benefit of due process, the right to a fair trial, etc. The right to a fair trial is a human right, btw.
And lastly, we do not have capital punishment in this country, so murdering anyone cannot be just.
How about you? Do you think murder can ever be just?