Neon Genesis Evangelion: the 17th Angel, Tabris, AKA Kaworu Nagisa AKA the Angel of Free Will
Here's an example of an age-old moral dilemma hypothetical: if you knew that pushing a granny in front of a train would save everyone on the train while killing the granny, would you do it? Would you cold-bloodedly murder an individual or a handful of people if you were certain the alternative was death on a far larger scale?
Normally these questions tend to fall apart when analyzed as actual scenarios. Why do I need to push the granny, specifically, to somehow prevent the train from crashing or derailing or whatever? Why can't I push something else in front of the train and just... not commit murder? Why can't I choose to jump in front of the train, since a single human body is all that's needed, and go with suicide rather than homicide?
Note that this post involves spoilers of the original Neon Genesis Evangelion TV series.
I'd argue that, in Neon Genesis Evangelion, the fight with Bardiel is pretty much exactly one of those real-life scenarios that doesn't work for the "main question", made into an episode: the apparent situation is that Shinji must murder an innocent human being to stop Bardiel, but really, who says so? Can't Unit 03's Entry Plug be removed somehow? Can't Bardiel be separated from Unit 03 without killing 03's pilot? Surely there must be something that can be done, something humane? The situation is ambiguous on a number of levels, and Gendo's decision to activate the Dummy Plug and make Unit 01 kill Bardiel/Unit 03/Toji, overruling Shinji's refusal to commit murder just because it might be necessary to prevent the extinction of the human species, can be viewed as gratuitous and callous, and indeed is viewed that way by many Eva fans.
Ah, but then we get to Tabris, and he clarifies, he simplifies.
First and foremost: Kaworu is an Angel, whom is a human being. Tabris is not some malevolent force possessing the poor sap known as Kaworu, such that we can somehow exorcise Tabris from Kaworu. No, a human being, a person, must die for the Angel to die, period. The series has long since established that Angels cannot coexist with humanity. If they aren't killed, humanity goes extinct. Bardiel could theoretically have been separated from Unit 03/Toji, and then Bardiel killed. Not so Tabris from Kaworu. They are one and the same.
As such, Kaworu's death is necessary for the continued survival of the human species. This is not ambiguous. It cannot be argued. It is fact.
Second: Kaworu befriends Shinji before the confrontation. This is important, because humans, as a whole, default to not really "counting" anyone they don't personally know and care about as a human being. It's much easier to justify murder, particularly to yourself, if you don't think of the murdered as human beings.
Kaworu befriending (Or seducing, as it's viewed by many fans) Shinji bypasses a psychological self-defense mechanism that could dodge answering "the question" being presented. If Shinji chooses to murder Kaworu, he is doing so fully cognizant that he is killing another human being. He can't pretend to himself that he didn't really kill a real person.
Third: There is no cavalry coming. Kaworu blocks all access to the room once it's just him and Shinji (As far as either of them knows, anyway. Rei is present for whatever reason, but Shinji and Kaworu don't seem to notice her) using his AT Field, and the series has long since established that AT Fields cannot be bypassed or destroyed by conventional tools. You need your own AT Field, which means you need an Evangelion... and Unit 00 blew itself up last episode, Shinji had to defeat Unit 02 immediately prior to Kaworu presenting him with the decision to be made, Unit 03 was destroyed in the Bardiel fight, and anyway Shinji is the only Pilot available right now as far as Shinji knows. Reinforcements are not possible, and Shinji and the audience both know it.
Shinji must make this decision. He can't stall and hope the cavalry come, taking the decision out of his hands. Make the decision, Shinji. Kaworu's blood on your hands, or humanity's blood on your hands?
Fourth: No contact is possible. Kaworu's AT Field is blocking not only the possibility of reinforcements, but also the possibility of communication. Gendo is not going to jump on the radio and order Shinji to murder Kaworu, now! Neither is Gendo going to activate the Dummy Plug system -Kaworu has disallowed these possibilities.
Shinji, years down the line, is not going to be telling himself "I was only following orders". Nor will he be able to tell himself "I didn't really do it, it was the Dummy Plug system. Gendo killed Kaworu." If he murders Kaworu, he murders Kaworu. Shinji can't perform the act and then pretend he didn't make that decision. He can't hide behind excuses.
Fifth: It's not a fight. There's no immediate threat. Shinji quite literally has Kaworu gripped in his giant death robot's hand, helpless, weak, human, while Kaworu simply sits there with a faint smile, waiting for Shinji to make his decision. Kaworu doesn't make a threatening move, doesn't provoke some defensive instinct, and therefore doesn't cause Shinji to accidentally murder Kaworu. He just waits. Patiently.
If Shinji kills Kaworu now, he won't be telling himself "It was an accident! I didn't meant to! I'm so sorry, Kaworu." Nor can he tell himself that it was self-defense, that Kaworu was attacking him and he defended himself, and it just happened to be lethal. If Shinji kills Kaworu, it will be cold-blooded. It will be murder.
Either Shinji murders Kaworu, and does so in cold blood, or he refuses to do so and humanity goes extinct. The end.
I think Kaworu is, quite simply: "Shinji. Will you become a cold-blooded murderer... or will you become complicit in the complete genocide of humanity via an act of cowardice, by running away, again?"
Tabris is labeled the Angel of Free Will, and I think that's rather apt for this scene: Kaworu, Tabris, Angel 17, whatever? He reduces it down to "Shinji, you must make this choice. Can you murder someone to prevent worse?" All the excuses and alternatives are cut away. No dodging the decision. No failing to give an answer. No having the decision made by someone else. You must make this hard decision.
I consider this the crux of the episode, the primary point and the thing that gives it its power. A lot of stories try to ask this question, in general terms, but it usually works out more like the Bardiel fight -too muddied by complicating factors to interpret a character's decision in the scene as being representative of their thoughts regarding the intended "question".
A subtle consideration: Shinji is that rare character for whom "do this or humanity dies" is a real choice. After all, Shinji finds suicide a tempting option, an escape from everything. The scene carries a lot of tension as a result: with most characters in most stories, a scenario in which murder is necessary to avert the extinction of the human species is one where they're very obviously going to go the murder route. They might dislike it, they might loathe it, they might have nightmares for years afterward and hate themselves for doing so, but they're going to do it because their own death is unacceptable, or because they're not willing to let their friends die to avoid killing this person, or... well, any number of reasons.
Shinji? Shinji isn't sure he cares about anyone else's survival, and he's not sure he wants to live. He could believably go either way.