Spoilers ahead? Careful reading?
Seriously like actual spoilers ahead ok???
Like seriously spoilery also is hugely theory-work and not really based on much fact.
I feel like there's more to this than just being symbolism. Everything outside the basement seems to more or less be grounded and more real. There are actual people outside the house that Embry talks to. They are actually going places. And D. Light seems like they are a real person with genuine concern, and is not some kind of detached symbolic representation. But whenever things come to the basement, is when it seems like things get a bit more muddy. Even when we "truly" see the basement, it turns out to have been a dream. Beyond that, we never see anything. The only real thing that tells us the basement is actually real is D. Light definitely has a real reaction to it, to all the locks, to how dark it is, and acknowledging that there is indeed a basement behind that door. After that, all we know about the basement is what Embry tells us. A monster appeared. Possibly ate their parents, or something. Trying to hold it off, and feed it. They mention meat seems to be more satisfying for the monster. That's about it.
I think it's possible that Embry seeing themselves as a monster is more than just symbolic, there may actually be something more going on than that. We never even see any bodies, and whatever happened to D. Light was clearly more than just falling down some stairs. Their face becomes grossly disfigured (admittedly, we never see their face before they fall down the stairs anyway so that's a bit up in the air) and there is a big and quick blood splatter. I think this may imply that whatever Embry did, was at the bare minimum quite bloody, and I think it's possible it didn't really leave much of any traces left, or what traces were leftover were severely mangled and possibly heavily disfigured. Perhaps we don't see any bodies, because there aren't any. There wasn't much of anything left. Why would that be the case, though? Because a monster did suddenly appear. And did actually eat them. And I think it was Embry. I think something happened that caused them to "lash out" if you will.
The basement may have come to more symbolically represent what Embry thinks and feels, but it seems quite clear it was chosen as a representative location because that's where the original incident happened. I don't think their parents just fall down the stairs. They went downstairs, and then something happened. Something killed them, somehow. A monster appeared, and ate them. The monster in the basement tries to mimic their parents? Unsuccessfully. In the dream, they clearly think they are the monster. I think that is both symbolic of regret for something they did but also literal. I think D. Light's case also shines an interesting light, revealing some details of what may have also happened to Embry's parents. They don't seem sure whether or not what happened to D. Light was an accident. They don't even seem to be sure of what actually happened. The dream Embry seems adamant they "pushed them down the stairs" while Embry themselves seems adamant that they were trying to get them away from the basement. I think both things were going on, and ultimately the "push down the basement" won out.
One of the weirder key details is that Embry seems to look VERY malnourished. Their hair is possibly suppose to be blonde? But it is now white. They are very pale. If whatever is in the basement is suppose to represent some kind of mental monster that she is feeding, is she feeding it literally or metaphorically? Well, I think it still seems right to think literally here, the preparation of meals happens outside of the basement. I think they are *trying* to feed themselves, but it isn't working. It isn't satisfying and isn't the nutrition they need. Except meat. This is why the monster reacts so viscerally and seems to be trying to get out. Nothing is working, so that subconscious part of them is clawing away trying to get out and assert control by asserting survival instincts. This may even explain what happened to D. Light. They opened the door. They tried to show whatever they are burying away. And as a result, D. Light "fell down the stairs", ending up in the basement where something happened to them just like Embry's parents.
One more thing: An interesting little detail that I think helps tie this all together in a neat little bow. Embry's eye. On the title screen. Glows a bit? Just a little bit, but it clearly does. :)?
I've left out details about the dad because I'm not really sure what that is all about, but I think some other comments here and on Manly's video can kind of fill that in. The dad is posibly some kind of monster or demon himself?