(1) That isn't a defense to murder in the US, and never has been.
(2) If you believe the New Testament is Scripture, then that principle is no longer morally operative (Matthew 5:38).
(3) Even if this principle were legally and theologically current, Byron Smith violated it.
She did not shoot him five times. She broke into his house and didn't actually take anything. The proper "eye for an eye" revenge would have been for him to break into her house.
Whether it was merciful or not, if the pair had not been in his house illegally, they would not have been shot.
Correct. What you are missing is this: you seem to be well aware that these teens were independent moral agents who had control over their actions and made their own deliberate bad decisions - yet you simultaneously maintain that Byron Smith was not an independent moral agent, that he had no control over his actions, and that he was incapable of making his own decisions.
The reality is that Byron Smith was in complete control of the situation from start to finish, and he deliberately and carefully planned to act immorally and to break the law.
Anyone who takes the Christian Bible seriously should be filled with horror at his actions.
Kill them now, kill them slow or fast, I don't care, but kill them. Don't talk to me about shades of gray or "Taking the Bible seriously."