Okay so chat posts are not the best for reblogging with commentary but regarding this:

Even
in the Pacifist run, there are some fights you can’t get out of. The
important thing is 1. to recognize which fights are which, and 2. to
spare even these enemies

once they are no longer threatening you. You can’t spare Asgore or Flowey until you’ve fought them, and the Omega Flowey fight in particular is set up in such a way that there’s no question of trying until after the fight is well and truly over. This latter is especially noteworthy because it’s the one time in the game where you’re in a similar position to the monsters fighting a No Mercy human.

Undertale is about power in a way I think often gets overlooked. You have the power to SAVE. That means you’re never in any real danger in these monster fights. Sans even makes a point, in some neutral endings, of saying self-defense is fine- but was it really self-defense, for you? Whether looking at it from the perspective of Frisk, a hidden Chara, or the player themselves, the answer is no. If you die, you come back unscathed. If a monster dies, they’re just dead. The monsters also have a real reason to fight. They have a long and very serious list of grievances against humans, and are suffering from human action against them even if the humans themselves have forgotten the monsters exist. And when, in the Pacifist route, do we face unavoidable fights? When we face entities with tremendous, genuinely threatening power. A king who knew he was doing the wrong thing, chose to kill, and succeeded six times, in the process teaching the entire underground that this was the only way to deal with humans. An entity who killed without remorse, for fun, over and over and over, threatening literally the entire universe in the process. You reach both of them eventually, but it’s not by playing nice and showing them how nonthreatening you are. Flowey, in particular, you have to confront forcefully just in order to make him listen to you at all. Until you do that, he’ll keep dismissing you as weak and stupid.

In the No Mercy route, you are the power that must be fought because it can’t be reasoned with. I think it’s worth noting that Undyne Undying is the very next boss after Papyrus, and Papyrus is practically the spare option personified in his own fight. Papyrus is a kind of point of no return, where you can’t possibly pretend not to understand exactly what you’re doing. Immediately prior to Undyne Undying, you get a kind of confirmation of that point of no return in Monster Kid.
In No Mercy and No Mercy alone, Undyne is an unambiguous hero. At no point is her violence towards you considered anything other than justified (unlike in other routes, where it ranges from “understandable but not especially heroic” to “wow that was messed up”). That’s because of your own unambiguous serial murder.

You are not an entity that can be pacified, so long as you hold power without consequences; therefore, there must be consequences. You will kill literally everyone if not stopped; therefore you must be stopped, period.

As a moral, this means that when you are the one with the power, you have a moral obligation to seek peaceful resolution when you can. However, when you aren’t, you have a right to defend yourself. Undertale leaves us a challenge much more complicated and difficult than “Never hurt people, because it’s bad”. It insists in strident terms that every life is valuable, and leaves you with the sometimes-contradictory task of actively defending all those lives to the best of your ability.
Everyone has a right to live- not only your enemies, but your friends, and people you don’t know, and you. Defending that right means avoiding
killing whenever possible, but it also means not meekly accepting your
death or the deaths of others.

In other words:

“Don’t kill, and don’t be killed.”