I think that in reality everyone is capable of reaching this point; it's just a matter of what has to happen to or around them to push them to that point. And if you are cynical you could sum it up like the judge did during the trial of my best friend; no one knows what a person might me capable of doing and anyone who who says that this man could never do something like this, is deluding themselves
However for those that reach this point and seem to stay there; well that would be insane.
This is why it works better if there are circumstances that lead up to the whole event, though that seems to fall in line with a familiar trope.
I've recently read a number of books whose main characters do this a lot and directly from the start. Which is also becoming a trope.
I think if you have a character like this then you need to craft a very plausible reason for them to be the way they are. I also think that the average reader will see easily that they do not seem to act normally; at least until they see that plausible reason. And if they are like me they will be put off if there doesn't seem to be any underlying reason for their abnormal behavior.
On the other hand if you chose to show that they are clearly insane then you don't really need to show much other than to make them a consistent nut case. And in that instance you would never see them as trying to examine their behavior at all since it seems perfectly acceptable and normal behavior.
I know this isn't as extreme as your supposition::
He raged, his face grew flush and in dim lighting Dan's shadow displayed clenched fists, he silently cursed everyone from his father, to teachers, to bosses who all felt so liberal about pointing out accountability; though each being reluctant to allow him the authority necessary to mitigate responsibility. Almost of their own volition his arm scooped down, and his hand grasped the empty six-pack of bottles. Without much thought he hurled it the full twenty feet past the desk and into the far wall. Bricks shattered glass and the shards sprinkled outward. An animal like growl squeezed out his gut. He stood motionless.
Grasping the broom from the near corner with his throwing hand, he whisked up the dustpan with his other and stomped methodically across the cement floor, each step delivering frustration into the unmovable surface, until he'd scuffed through glass, with a gritty grinding sound. At the far wall he commenced sweeping up against the wall, collecting all of the pieces from the open floor to the areas under the chair, scraping glass interrupted by sounds like chimes; until he'd reached the outer limit of the event. His shoulders relaxed in slow progression throughout, until he bent slightly to use the dustpan to collect all the bits. He glanced around momentarily. Then with exacting swift movements he dropped the debris into the trashcan.
Quietly he turned and calmly walked back to return the tools to their proper place.
Only then did he notice the young clerk, Clarisa, and he pondered the puzzle of how long she'd been standing there. Her expression his only tell. She seemed to watch his every move with an appearance less of fear and more of bewilderment. As though she couldn't imagine someone going from red hot to mellow in such a short span. It was always like this for him. And he wondered if he could just sustain anger long enough, perhaps he could stand up for himself. For once.
::But could be considered unusual behavior.