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The first one I believe was Moirai.
(I will confess that it took some digging to find that name, as despite my memory of the game itself I was blanking completely on said name, and my searches at first turned up nothing! ^^; )
My Games and Asset PacksThe second game might be https://tvtropes.org/pmwiki/pmwiki.php/VideoGame/NierAutomata
? The ending sequence of Automata does something very similar to what you described.
please excuse my terrible links, it's been ten years since i last used this site...
Edited by gracksaExcellent! ^_^
From what I gather, the ending sequence of Nier: Automata is actually a scrolling shooter. Whether it's retro-styled or not I don't know, however.
It might thus be worth looking up gameplay footage of that specific ending—not the main of the game, or any of the other endings—to see whether it might not be what you're thinking of.
My Games and Asset Packs

Now, the two games I'm going to mention are completely separate, only being connected because they both have choices at the end of the game.
The first game was some web game where there's an old guy you can choose to spare to kill, as well as asking them questions. This person turns out to be the last player, and you will become the old guy for the next. There's also a girl you can Mercy Kill, but even if you don't choose to, you get blood spilt all over you.
The other game, I remember way less of, only an end sequence where you are placed in a Hopeless Boss Fight until other players (who sacrificed themselves) say inspiring messages while helping you beat the boss, and afterwards, you can choose to escape or become one of them.