I want to make a dark seedy place where guards will never enter, despite violence taking place. How can this be achieved?

You could possibly have everyone in there be in a special faction with the "special combat" flag enabled so that guards are never called there.

Or set everyone's responsibility low enough that they don't report crimes.

I probably added an NPC that has high responsibility to my area where I've never seen guards before, I'll check that out, thanks!