Because it presumes guilt. Because it does not provide for a trial. Because it operates on hearsay. Because it is rule of the strong over the weak.
There was a great Hitchcock Presents episode. A woman is raped and her husband is taking her home after filing the police report. As they're driving a deserted road, she cries out: It's him! It's him! The husband asks: Are you sure? She gets hysterical. "It's him! It's him!"
The husband leaves the car and kills the man. He returns back to the car, satisfied that justice has been done. They start driving down the road and the woman sees another man.
She cries out: "It's him! It's him!..."
That's what happens with vigilantism.
That this even needs to be explained is disquieting.