...

Obviously the biggest difference between aitsf and aini is that aitsf is a "whodunnit" story while aini is a "howdunnit" story, but the second biggest difference is that aitsf is about people being chased by their past. If you left any of these characters on their own, they would be fine just chilling. If not for Saito, Date would have never found out about his past because he simply would have never looked. If not for Iris's cancer, Hitomi would have left Sou alone and Manaka in the basement. The cast is trying to let go of their trauma- it's just that your trauma will catch up to you eventually. Aitsf is a story about trying to let go of the terrible things that have happened to you.

Meanwhile aini is a story about holding onto the terrible things that have happened to you with both hands. You have witnessed Unspeakable Horrors six years ago and you need to eat someone's liver about it. Ryuki, Shouma, Amame, they're not being pursued by anything, they're actively hunting a bitch down with pipe bombs in their basements and if shit blows up then that's how it is.