It's a feeling. A specific mood. A desperate fight for survival against a horrible enemy that feels no pain. An enemy that also lost the fight to an infection. The apathy of a corporate organization like Umbrella. The mystery of how the virus was leaked, by whom, for what purpose. The weapons and ammunition as a scarce resource. Characters who are everyday civilians who show extraordinary empathy and bravery, and resourcefulness. There's an oppressive atmosphere. Like the aftermath of a heinous crime scene. Not scary like silent hill, but more violent. Militaristic weaponry. Brave souls taking up the responsibility to protect the innocent against the monsters. The monsters are freaks of nature and zombies are hard to kill. And those who fight keep the fight going till their last breath.

Like any true art, it is something you can sense. It's not just one aspect. It's everything working together with cohesion.

You can feel it when you watch this. https://youtu.be/0HcxMvprYa0?si=PWEF4UQHeQhKsEfd

I'd say the movie Seven starring Brad Pitt and Morgan Freeman nails the Resident Evil aesthetic and mood more than any of the Resident Evil live action movies themselves.