Fits of these with the ending and a certain scene involving a collapsing bridge before the final Colossus.
After following you silently yet with utter devotion, your only companion in the vast empty world. And to top it off, it was to save your bacon...
Here's an experiment. Try to look the Colossi in the eye before you kill them, then watch their slowmo deaths.
The Downer Ending sent this trope in tears as you try to fight a current to see if the girl you killed sixteen colossi for is okay. Then you shed tears of joy when she wakes up to find your loyal horse who's Not Quite Dead, and then as the credits roll, the camera pans over the decaying carcasses of all the Colossi. It was so simplistic and honest it was overwhelming.
What's even worse about the ending is that while you know that Mono's okay, Wander doesn't, since he's killed before she wakes up. He probably died thinking he'd failed her and that everything he did was for nothing.
YMMV, of course. There are all sorts of interpretations to this game. Mine? Wander sprouts horns near the end, and then turns into a Colossus, but it's implied that he may be able to someday atone for his sins. This may stretch believability in terms of how quickly it could happen, but at the end, Mono finds a baby with horns who could be Wander! So you could be crying tears of joy. Maybe.
Wander's pained face and outstretched arm towards Mono after one of the guards pierced him through the chest with their sword. He's either losing his own sense of self or dying, yet all he apparently wants to do is reach her.
The scene where Wander dreams about Mono coming back to life. Ten times more wibbling when he actually mumbles her name before waking up.
This troper started to tear up when he got to the end and fully realized that Wander, and by extension, the player is the real villain in this story!
I got teared up when I realized the villain (who, from my perspective, is Lord Emon) actually won.
Killing Phalanx is something of a traumatic experience, mainly because it never does anything to attack Wander, ever, even when he starts firing arrows at it and stabbing its weak spots.
It can happen first with Avion. One thing is to defeat a foe that starts attacking as soon as you are in sight, but Avion starts attacking only after YOU attack him first. The moment you see him crashing into the waters there is a high chance you'll begin to seriously ask yourself "What have I done?"