In William Shakespeare's Titus Andronicus, the title character kills his youngest son, Mutius, in the first scene for very little reason and with amazingly little fanfare — no dying speech, no nothing. He is called out by his remaining sons.
Similarly, in Hamlet, when Hamlet switches a letter ordering his own execution with one ordering the deaths of its bearers (who happen to be his erstwhile schoolmates Rosencrantz and Guildenstern), his friend Horatio calls him on it. Hamlet, however, brushes it off with a glib, "They are not near my conscience," which Horatio seems to accept.
At the climax of Shakespeare's The Tempest, Prospero is actually talked out of his revenge plot when his servant Ariel describes the distress of his captured enemies. Prospero realizes that if Ariel, a spirit of the air, can feel pity for these men, then he as their fellow human being should be compassionate as well.
Wagner's Parsifal actually introduces its eponymous hero this way, with him being reprimanded for senselessly killing a swan. Of course, he's The Fool and has a lot to learn — he doesn't even know his name at this point.
Fairy tale conglomeration/parody Into the Woods is -MADE- of this trope. The entire first act highlights the amorality of various fairy tale favorites in their quests to find 'happily ever after', with a character occasionally calling another out on it. It all comes to a head in the second act, which is dedicated entirely to the various characters dealing with the ramifications of their cutthroat methods. The song 'Your Fault' is a half example of this, tempered from a true 'what the hell' by the fact that the characters are simply scrambling to keep the blame off themselves, which ends in the unanimous decision to blame the Witch. Her response is of course a verbal spanking of epic proportions in 'The Last Midnight' for, aside from their amorality to begin with, wasting time finding blame when they should instead be dealing with the damn problem.
Though the audience may find themselves thinking the Witch has very little room to talk, what with her abuse of her adopted daughter Rapunzel (even if it's ironically rooted in love), which comes up as way worse than, at least, Cinderella wanting to escape her unhappy home life. (Cinderella's only amoral action, in fact, is participating in blaming the Witch with the others.)
In Henry V, the king goes incognito before the battle of Agincourt and mingles with his common soldiers. Unsurprisingly, they don't have a lot of cheerful, positive things to say about the next day's battle, since they are vastly outnumbered by fresh French soldiers, and they are not thrilled about the king that led them there. Henry (still pretending to be a regular dude) takes some offense and formally challenges one of the men. After the battle, he (now fully visible as king) calls the guy out, and the guy tells him if he got angry at his soldiers' honestly expressed thoughts, it's his own fault for dressing up like a commoner to fool them in the first place.
In Wicked, Elphaba calls out Glinda for using her sister's death as a trap to have her arrested. Though Glinda didn't intend to for Nessa to die in order to lure Elphaba, she did suggest using Nessa as a way to get to Elphaba.
Elphaba: I can't believe you would sink this low! To use my sister's death as a trap to —>capture
Glinda gets to deliver one to Elphaba at the end, when Elphaba is holding Dorothy hostage
Glinda: I know you don't want to hear it, but someone has got to say it! You are out of control!
When Adams and Jefferson want to keep the anti-slavery clause in the Declaration, Rutledge of South Carolina launches into "Molasses to Rum," an absolutely damning song where he points out that New Englanders are the primary carriers of slaves from Africa to North America and it's absolutely hypocritical of them to blame the whole thing on the South. Then he walks out with the entire Southern delegation.
Shortly after this, Franklin tells John that the anti-slavery clause must go, and Adams insults him. Franklin sharply corrects Adams' Surrounded by Idiots attitude, pointing out that the other congressmen are as proud and accomplished as Adams and they, like he, were given the trust and responsibility for their own colonies.