I've told this story before but for me it was when I was around 14 or 15 years old. I was playing Fire Emblem Shadow Dragon for the Nintendo DS. This was in fact my first Fire Emblem game so I didn't really care much whenever a party member permanently died. I basically ended up brute forcing my way through the entire campaign with only Marth and a whole team of generic units the game had to give me because literally everyone else was dead. Even at the very last map when you have the option to bring someone back to life, that person ended up dying in the same map anyway.
And still I didn't care. I took down Medeus and congratulated myself for finishing another game. That is until, I got to the credits. See most Fire Emblem games use the credits as a sort of epilogue to see what each character did after the war. Well since I led everyone to their death I was treated to wave after wave after wave of "[Blank] died in Chapter [X] and was lost to the pages of history…" Over and over and over again. And to really hammer it home, once the credits are done there's an epilogue scene where Marth, the only character that has to be alive by the end of the game, was mourning the loss of his potential girlfriend Caeda. I didn't realize she was even supposed to be his girlfriend! I let her die to arrows as soon as I recruited her! This was the moment where I broke down crying. It was my awful tactics that led to Marth feeling such great sorrow. They didn't deserve to die, I did! The guilt of my actions weighed on me so heavily even though they were mere sprites and pixels in a glowing box. This is the moment I first remember really thinking I had to kill myself for what a monster I am. I know it sounds like a joke but to me, it was deathly serious. To me, this is the start of me putting suicide on the table and since then it's always the solution I feel necessary for my evil actions that have hurt real people.
Now I ended up trying a new save, this time with an Action Replay device so I could make it all the way to the end and have everyone live, although there is that canon death where you have to choose between two guys and no matter which one you choose they can't even be brought back to life in the end.
Anyway, that's why I always play Fire Emblem games in casual mode now.