The entire process of jurisprudence and prosecution is just as beset with pitfalls as is the lamentable and detestable practice of taking responsibility for someone else's life. I don't mean this in the sense of soldiers placing their lives between their homes and the war's desolation, but in terms of a fearful, passive person abrogating their own personal responsibility and demanding that another take care of them -- as is commonly done by the hordes of "sheeple" who require that police and "authorities" guard them 24/7 from the risks of the world.
The further one gets from the exact instance of the offense, the harder it is to prove with 100% certainty who did what, with which, and to whom. It is the nature of causality, entropy, and the transmission of data that the chance of error will increase in the equation as soon as more than two nodes of causality -- the aggressor and the defender -- are introduced. At the moment of the offense, the defender can prove, with one hundred percent accuracy, who is doing it. They are also the best and most perfectly placed person to take effective action to stop the attack, and if necessary, to administer sanction up to and including terminal force. They are the ONLY person who can do this and are in any way able to be 100% certain of getting the right person.
Once you add other persons to this situation -- investigators, prosecutors, defense attorneys, judges, juries, grand juries, reporters, and the vast unwashed masses of the vox populi, you have just opened the door for massive error at every single layer. Investigators did not see what the defender saw, and could get the wrong person. Police might go to the wrong address, break down the wrong door, not have a perfect warrant, improperly Mirandize the suspect, "tune them up" and violate their civil rights, question them without counsel...attorneys might commit all manner of errors, from basic incompetence to violations of civil rights to outright corruption. Judges might be politically biased, half asleep, incompetent, senile, or insane. (Woe betide you if you go to court in upstate New York -- your judge might not even be qualified, or might pull out a violin to play you a sad song when you make an objection.) Any level of this mob might be up for election, under investigation, fighting an indictment, under scrutiny, or otherwise in the public eye, and trying to make a name for him or herself, or worse, trying to hide from it. And this doesn't even take into account the possibility that the aggressor might have powerful friends -- his Daddy may be a Union boss, and the current D.A. might get significant re-election money from him, and the case may simply go away due to "lack of evidence" or "technicalities". Cost to the public, depending on how much of a circus it is, between 1.2 million and 3 million dollars, all passed back as taxes. This does not count your personal costs, when the aggressor and his rich, Union-boss Daddy sue you for defamation of character.
Or you may get lucky, Klono and Noshabkeming may smile, and the guy may actually be found guilty. The trial, and his projected incarceration costs are on the order of $450,000 to the local government, which will be charged back to the public in the form of taxes. He appeals. The circus starts again, with the evidence even harder to get straight.
Or he may only get 15 months for a plea bargained lower offense, out in 6 for "good behavior" or due to overcrowding, and you get your door kicked in unexpectedly when he comes to settle the score. This time, though, you've learned. You perform a radical encephalectomy using an ancient technique that employs a high-speed object propelled by expanding chemical gases in an enclosed space to accelerate the surgical device toward the material to be excised. (I call this technique projectile trepanning.) The perpetrator was instantly identified, cost to the public was $8.00 for a neoprene body-bag and $8,000 for one autopsy and related services.
Therefore, reason would seem to dictate that the efforts being made to prevent citizens from simply acting at the cusp of events on their own behalf are counter-productive and detrimental to the very society such efforts are intended to protect. Instead, citizens should be encouraged to act immediately, since they, and only they, are in any way equipped to know with PERFECT CERTAINTY who is committing such acts upon their person, and are the best placed persons to act to stop them. The "death penalty" then becomes nothing more than the "crime of stupidity" that has already been mentioned.