Gonna do a random analysis.
In theory, if you let Haru die, you won't be fighting Belial. Can't think of any important characters that would save.
Next is Keisuke and Mari. Do neither of their events, and you will kill two birds with one stone.
Missing Midori's event with the mob will probably get her killed.
Gin is... weird. He's connected to Haru and can (I think) possibly get himself killed if you give him info on Aya, but that event may not happen if Haru dies. I dunno.
Technically, Amane is also killable, but only on the 8th day Despot route. And Haru dying locks you outta that.
tldr: Not technically possible to kill all killable characters in Overclocked, but may be possible in vanilla DeSu.
