These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Moral Event Horizon: Joker takes a running leap over this when he murders Dick/Robin and Babs/Batgirl with homicidal glee. You can hardly fault Bats for killing him after that
Jimmy Olsen crosses it when he casually murders Kal-El's Amish foster parents
Narm: Batman having an Heroic BSOD that borders on DEH crossing after the deaths of Dick and Babs and his murder of The Joker is NOT funny. The ridiculous expression in his face when Alfred and Selina try to make him react is, though.
Selina dressing up as Robin to try and get him to come to terms with himself could also count