The spine is a very small target compared to the head, so your strike must be very accurate, not only in location, but you must hit at an angle that will dislocate the vertebrae far enough to damage the spinal cord.
That was my original point: that hitting someone in the head with a brick forces you to approach within a position where you can be attacked. A better solution is to simply throw a brick at his back, which would stop him, forcing him to stand. If he chases, you'd have a chance to flee, also giving the victim a chance to escape as well. There's really no reason to kill the guy. Better yet, call the damn police first, then hit him with the brick simply to delay him long enough for the police to arrive.
The legal factors will not even be considered at that time.
My point wasn't that the character would consider it, but that it's a potential plot hole, as readers likely will. You can address it after the attack, but the police and prosecutors will likely bring it up.
Would his attack on the perp be considered premeditated if he had come across the attack and flashed back to what happened to the other victim, causing an 'over-reaction'?
It's open to interpretation. If he's white and the person he attacked either black or colored, then there's an excellent chance he'd be let off. However, the cops and prosecutor would likely apply a LOT of heat, and charges are likely, although a conviction largely depends upon how easily swayed a jury is. But again, the issue isn't the character's thoughts, or whether he'd be convicted or not, just that readers will likely question his actions, so the author will want to address those issue at some point in the story, probably either in the after-the-fight cool down period, or when he's questioned by the police.
That's what a plot hole means, it's a problem with the plot which should be addressed somewhere.
Yep, and he could plead temporary insanity.
Except, temporary insanity is a terrible defense that rarely succeeds. It's mostly used by the obviously guilty in an attempt to avoid the death penalty and garner public sympathy. But it's only selected if there's little chance of any other defense succeeding.
Loss of memory would definitely mean he could not have known the difference.
No, memory loss implies he/they suffered from either combat stress or a blow to the brain, but that doesn't impact whether he knew what he was doing at the time or not. That's determined by his actions both before and after the incident. Forgetfulness is more likely their being terrified they'll admit something that actually happened. Instead, mental illness means you couldn't differentiate between right and wrong, or that you could differentiate between reality and delusions, which is extremely difficult to prove!