Yep I 100% agree it was never planned lol
Shit is so out of nowhere
Just to make sure, I began to re-read from the beginning and in chapter 6, Nezumi-chan falls asleep saying
"Thank you for finding me"
while Ao-kun is wide awake and immediately flashbacks to a younger Nezumi-chan killing some woman with the brother LITERALLY LOOKING LIKE YOUNGER AO-KUN.
7 years lines up with about the age of what looks like middle-school Ao-kun and Nezumi-chan then to collegeish age now. Also if they sent Nezumi-chan for the assasination, that means his sister would have connections to the underworld which would make sense of why Ao-kun was too after his sisters death.
Ao-kun "found" Nezumi on purpose. This has been planned since the beginning but I still think that Ao-kun did everything out of pure love of Nezumi. A real true-love at first sight.
This part is just my theory but my bets is on his older sister was his only but abusive family, fell in love with Nezumi for killing her, then got involved with the gang looking into Nezumi, and only allowed Ao-kun to get into a relationship/meet her if he was their puppet to play with her emotions.