Because vengeance has never done anyone any good. You never feel better after getting vengeance, just hollow. Thus, a good legal system should strive to provide justice, not vengeance.
To use the example from a sibling comment, if a person kills a child and the father kills this guy out of vengeance .. it will do those children good, who can now live in safety afterwards from that person.
But if in reality the murderer also had family who did not believe he murdered anyone in the first place now set out to seek justice/vengeance, then yes, it becomes a war .. which is why we have courts and police nowdays, but what justice is, is still rather arbitarily defined. Concretely it means enforcing the law. And laws are written by people.
Have you ever distributed vengeance so you can personally speak how you felt? Or are you mindlessly repeating strings of words that are supposed to go together like an LLM?
I don't have to justify to you, random internet stranger. I have made my share of experience, and read a fair bit about that of others, in history and literature; and I'm confidently standing behind my opinion.
To use the example from a sibling comment, if a person kills a child and the father kills this guy out of vengeance .. it will do those children good, who can now live in safety afterwards from that person.
But if in reality the murderer also had family who did not believe he murdered anyone in the first place now set out to seek justice/vengeance, then yes, it becomes a war .. which is why we have courts and police nowdays, but what justice is, is still rather arbitarily defined. Concretely it means enforcing the law. And laws are written by people.