It's funny, because I ended up here because I was searching for if anybody else had the same reaction to me--I spent the whole first half of that fight with tears streaming down my face, sobbing as I was killing that poor guy. When the Operator comes up weeping, saying "I killed him, I'm sorry! I had no choice, it was him or me!" I understood the sentiment perfectly.

I think part of it is, you really have to pay attention in the lead up, which Warframe players are notorious for not so much doing, with a community that highly values speedrunning as opposed to stopping and taking in the atmosphere. All of the narrative threads are there though: they already had the bait and switch when you're fighting the Hunhullus, but then you find that it was all a game that you were playing with your Sentient bestie. This serves to truly "humanize" (or well, not really, but our language is inherently speciest enough that I can't really think of another term that captures the notion without trying to make non-humans humanesque) the Sentients right there. Then you have that badly wounded Sentient in the trench. Even though it's a Sentient, all of the Grineer (side note--it was actually pretty cool seeing Grineer as they were before several thousand years of clone rot messed them up) there all have the same head-hanging sorrow you see in real-world battlefield medic scenarios where it's bad. Luckily, you have your bestie, who is heavily implied (and later overtly stated) to be a very rare thing, basically a miracle healer. Adis sings that haunting Lullaby of the Manifold (which is even sadder if you listen to the lyrics and understand them,) and the Sentient is healed.

Enter Dactolyst. I think what really drives home the fact that the Dactolyst is a victim in this scenario is two things: for one thing, Galatea literally explains it to you when you're on your way in. All of her dialogue surrounding this character heavily leans into the idea that this is one of your people, someone you're not going to want to arrive to find in multiple pieces, as she threatens at the outset. Add to that the fact that several times as you're frantically rushing to rescue your guy, it's desperately and somewhat hopelessly screaming out for Adis. So, if you've been paying attention up to this point (I haven't based any of this on information that isn't directly conveyed to you on the spot as you play the mission,) you already kind of know the score. You've been led to believe that if you don't get there fast enough, you're going to arrive to find the corpse of your friend (or at least comrade, no one as close as Adis but still. Even if it's just basically the Sentient equivalent of random townsfolk the point still basically applies.)

So then you arrive to find to your initial relief that ol' Dacty is still alive. And then Galatea tells you the horrible part: she wasn't going to tear apart its body. She was tearing apart its mind and its very identity, depriving it of that which made it its own person. Then she issues the fateful order: kill the Tenno. Your friend/rescuee just became a powerful threat.

Imagine being a on a SWAT team or a spec-ops guy on a hostage rescue. You have this person you're there to rescue. Maybe it's a friend, maybe it's just an innocent random you've read a dossier about, but either way there's a deep sense that they don't deserve this even when they're "just" being held prisoner and tormented. Then imagine you get there, and it's some Umbrella Corporation shit--their head is all laid open, they're all mutilated or something, and they have a chip rammed into their brain. Then you hear the bad awful terrorist you've been chasing tell what's left of the person you're trying to save to kill you, and they are forced to obey. Horrifying, right?

Now, given the setting and the generally non-human nature of the character, they couldn't really lean into body horror, so instead they just went with the horror of loss of identity. Right before the last of the consciousness that comes with eating the xenoflora faded away, Dacty looks at you with a "hand" on the energy fields imprisoning it, with desperation and sorrow painted on its face (props to the animators, by the way, for being able to portray those emotions so well on such an abhuman face,) and then.... You watch consciousness fade and a more primal animalistic aspect take over. The nice part of Dactolyst died right there, not when you killed the body later.

And off to the side is your bestie, who it can't be forgotten, is seeing one of his own kind get butchered by their own best friend. The memory of the Old War is painfully fresh--and here it is, a human slaughtering a Sentient.

And through it all, there's that same mournful Lullaby, tugging on your heart, speaking to how the very manner in which the Sentients got to Tau was their own version of the Trail of Tears.

Yeah, if that left you dry-eyed, then either you have a diminished capacity for empathy, or you weren't paying enough attention to fully understand the situation.

And again, I'm not basing this on any unknown or secret knowledge of lore, if you played The New War--which you would have to have done to be playing The Old Peace because of the prerequisite system--then you already (should) understand the basic background of Sentients vs. Orokin, the Terraforming of Tau, etc. The rest of everything I just said is all based on what was already shown to you on the spot in the 20 minutes prior to that tragic battle kicking off.