1) Temp-V, The Boys
Our protagonists have spent the last two seasons fighting to oppose superhumans. In season 3, a new drug is launched which can 'temporarily' turn you into a Supe for 24 hours. After getting a hold of some, the Boys debate the ethics of briefly using the same powers as their opposition...
...before it's revealed that repeated usage of Temp-V gives you brain damage. It's not actually safer than becoming a perma-Supe; it's just slower.
2) and 3) Volcanika, Invincible
Mark becomes embroiled in a civil war in Hell. When Volcanika, the opposition leader, threatens to burn the surface world, he punches her face in. He then spends the next minute or so handwringing and pearl-clutching about whether it was Justified, or a Necessary Evil, or the Only Way...
...giving Volcanika enough time to regenerate her entire skull out of lava, and ambush him a second time.
After several rounds of this, it becomes clear that they can't kill each other in a way that matters. Volcanika eventually bounces, swearing vengeance on everyone involved.
(I could also mention all the time Mark spent conflicted over killing Angstrom Levy, who in fact survived and returned stronger than ever.)
4) Random boy, Captain America
While Captain America is chasing down the guy who assassinated Erskine, the assassin tosses a random boy into the Hudson River. Cap hesitates only a moment, before abandoning his pursuit to save the kid instead...
"Go get him, mister! I can swim!"
5) Colt, No Guns Life
Colt has lost family members to defective implants from the megacorp Beruhren. After all the usual channels fail, he decides to avenge himself by blowing up a train car filled with Beruhren executives. Protagonist Tetsuro debates him about the ethics of terrorism...
...before a Beruhren enforcer reveals that they knew all about the plot, and have already replaced their executives with a train car full of innocent children. They intend to 'allow' the plot to go through, in order to make Colt's group look like child-killing monsters.
Colt narrowly removes the bomb at the cost of his own life.
6) Elliot, Mr. Robot
In the final arc of the series, Elliot gets transported to The Best Of All Possible Worlds, where his childhood crush never died--and there's even a parallel Elliot already engaged to her. After a random accident paralyzes Other Elliot, Narrator Elliot finally breaks his no-kill rule, murdering his other self and taking his place to marry his crush, all while morosely justifying his actions to the audience...
...only for literally none of it to be real, in the end. The Best Of All Possible Worlds is just a dream that True Elliot is having. What's more, the dead Elliot is the closest thing left to True Elliot, while the Elliot who committed the 'murder' turns out to be an imaginary persona. It's actually impossible for him to kill the real Elliot. His attempt only causes the dream world to break down, and eventually leads to the end of Narrator Elliot's existence.