I understand you track the actual damage not the overkill damage the heroes do to the enemies... But on pg. 21 of the rulebook it also looks like the damage the heroes get from the enemies increases threat.
Rule book is a little mess I think, gunna kinda suck later if they update it and we either have to print a bunch of stuff or pay $$$ for a correct cleaner version.
The rules say damage received by darkness, or damage received by enemies.
Not damage received from darkness or from enemies.
Only the damage the hero doesn't on monsters give him threat.