i am sorry probably i missed it, can someone give me a clue why ruby knows that aqua looking for the murderrer?
Late reply but:
As shown Ruby (who's arguably a bit slower on the uptake than Akane) didn't sleep this night but instead was grinding her gears.
Since Aqua never tried hiding he (supposedly) doesn't like acting, yet still worked for the director and participated in various works there must've been a deeper meaning behind things.
The incident (finding the corpse) gave her the push to pursue and think about this reason of Aqua's a bit more.
And with the small hint given by Akane she arrived at the right conclusion.
Maybe the weird crow girl told her something more or it's siblings having similar thoughts or whatever.
I think she was having thoughts of revenge now that she learned the stalker didn't act on his own (a thing Aqua figured out years ago).
And when she started planning her revenge (aka how to find the one responsible, I don't think she has identified them as their father yet) she realised her plan showed similarities to her brother's actions.
Anyway, that's my take