The manga is an adaptation of a light novel, and is about an entire high school classroom getting transferred to a fantasy world. The protagonist was a weak guy who got attacked by a classmate and fell to a dungeon. As he tried to escape the dungeon, he gradually became stronger as he defeated and ate dungeon demons.

Dungeon Seeker

That motherfucker is on a path of destruction and revenge.

Arifureta Shokugyou de Sekai Saikyou

Definitely Arifureta, but Dungeon Seeker has also a similiar but more dark premise.

Seriously? There's another manga where a guy gets fucked over by his friend/love interest after being transported to a fantasy world and thrown into a dungeon where he becomes stronger by eating demons he defeats?

Not quite, that's why I said Dungeon Seeker is darker, in Arifureta he got fucked up by... well, I would spoil you the story If I answered that x'D.

Thank you very much guys. It was indeed Arifureta.
I lost a shitload of browser data a few weeks ago and I'm currently having a hard time trying to remember which mangas I didn't add to my reading list.
I hope you guys have an awesome and successful 2017.