Playlist of all Kill Everyone Challenge runs here: https://youtube.com/playlist?list=PLGjSlmbjQXnzmszPcthRgb3muehJVSnUX

Here is my go at killing all NPC's in the Haven Island map silently on Master difficulty. Ending up with -1,060,000 on the "non-target kills" category divided by -5,000 per kill would be 212 non-targets killed and 3 targets for a total of 215 NPC's killed silently.