This might be the most common interpretation of the movie. At the start, the kids’ father can be heard telling someone on the phone that Kevin fell down the stairs but was, “Fine” and, “Didn’t need stitches.” Shortly after this, the nightmare begins and we’re left to wonder just how “fine” Kevin really was.
Redditor “DCBBF68” suggested that, rather than being in hell, “Kevin isn’t dead so much as his brain is slowly dying,” and we’re witnessing the slow degradation of his mind — hence the increasingly surreal imagery. Other Redditors have reached similar conclusions. According to “Termmie,” Kevin “goes comatose after [the] incident, and we stay inside his comatose dream for 574 days.” The child eventually “forgets the layout of his house until he can only remember the TV. He forgets what human faces look like. Eventually he forgets his name.” At the end of the movie, a barely distinguishable face emerges from the darkness. Kevin asks its name, only for it to reply with the same question. We don’t hear Kevin answer — hence the idea that his comatose mind has been fully depersonalized.
The coma theory is one of the more likely alternatives, but as “DCBBF68” points out, it would require that Kaylee is merely a figment of Kevin’s deteriorating mind, rather than an autonomous character — “Things happen to her when Kevin isn’t there ,” suggesting she and Kevin are going through the same experience, therefore it can’t simply be taking place solely in Kevin’s mind. Whether it’s just Kevin or both he and his sister, the whole thing is not only incredibly creepy, it’s devastatingly sad in the sense that this quintessential evil is being visited upon two young children incapable of grasping the enormity of it all.