No way to make such a mod. "Freekilling" is entirely contextual; it's not something a script can accurately determine or nitpick.
For example, a player could be refusing to follow the orders of a guard and have no weapons, and gets justifiably killed. Under an ordinary freekill mod like the one you linked, that player would be considered innocent. The script doesn't have the capability to understand situational context; that's an incredibly complex thing that requires human-tier intellect, something a simple blockland script will never be able to do.
The simple fact is, Jail RP, like CityRPG, is a fundamentally broken gamemode that relies heavily on an honor system to work properly, as well as requiring constant and intense admin supervision for it to not devolve into total chaos. This is why you see it being the focus of so many server-related dramas, because it has a huge capacity for abuse. Freekilling can't be reasonably prevented, it can only be dealt with after it's happened.