Do you know any anime where the MC is betrayed by his allies/friends and becomes ruthless/dark?
Also during this there is a romance story between the MC and female character with a similar pesonality?
Kind of like Arifureta only that it isn't a harem story...
Do you know of any Isekai Anime where the MC gets stuck in a fantasy world (preferably) and uses guns?
I only know of
Arifureta Shokugyou de Sekai Saikyou
Isekai wa Smartphone to Tomo ni
Death March kara Hajimaru Isekai Kyousoukyoku