Jung's "shadow" is an archetypal construct heavily laced with Freudian archetypes and methodology, including Freudian "Ego." None of that is how modern psychology, based on modern neuroscience, views how the brain works.
Jung's "shadow" starts off as an unknown negative and malevolent element of a person. Almost a counter-personality. In the beginning, according to Jung, the person doesn't really realize it's there and only glimpses it through others, sort of like a reflection. Until, for some, eventually, they are able to recognize and come face-to-face with their own "shadow" and either control it or integrate it.
None of this matches with the best current neuroscience or psychiatric practices. It works as a metaphor or maybe a construct in order to work towards a goal, sort of the way I describe the "Robodroid." But both Jung's "shadow" and my "Robodroid" are not actually the nuts and bolts of how the brain works. "Robodroid" is a metaphorical construct to model how to get the desired responses from the hippocampus and functional working memory portions of the brain when awash in epinephrine and norepinephrine. There's not actually a "Robodroid" but it helps describe what the inputs and outputs look like to folks. The same sort of thing with Jung's "shadow." There isn't actually a portion of the personality which is violent and negative. It's just your personality and responses, some of them "programmed" through experience and trauma, some through "instinct" (such as selfishness), and sometimes even through physical trauma to the brain such as a TBI. Most of it is about how the brain constructs neural pathways through experience and stress, usually as a survival mechanism. But it can be a useful metaphor to work with what you perceive as your "negative" side. I have a Marine buddy who suffers PTSD; he refers to these negative responses and that element of his personality as "the man in the box."
What surprises me is how much I'm suddenly hearing about Jung's "shadow." I think he came up with the idea for it somewhere between WWI and WWII and, like Freud, he's considered a pioneer but psychology has moved past many of his ideas. That I'm suddenly hearing more and more people talking about Jung's "shadow" means... well, something. I'm not sure what it means yet, but it means something. There's a renewed interest in understanding, integrating, and controlling the violent nature of humans, or at least individually. Might be related to the impact of the social violence from the political and COVID riots from the last few years on the collective psyche of the world, ... or something.
Peace favor your sword,
Kirk