Actually searched for manga/novel, where the mc turns evil cause of certain tearjerking events, yet I found nothing so I'm asking you:

"Do you have any recommendations in mind?"

I'm actually just searching something to read since I read the Mushoku Tensei novel, where I somehow wanted him to turn insane and become the new demon lord.

Cant go wrong with Berserk
Mugen no Juunin , Koroshiya 1, Dorohedoro , Shin Angyo Onshi

Manga:
Akazukin to Yasashii Ookami

Novels:
Senketsu no Elf (Novel)

Shuumatsu Nani Shitemasu ka? Isogashii Desu ka? Sukutte Moratte Ii Desu ka? (Novel) - it's not 100% tragic, though. The overall tone of the story is melancholic, with sad events and bits of hope intertwined together.

As for your search for stories where MC turns evil, check Katahane no Riku (Novel).