¯\_(ツ)_/¯

He lost both his parents due to an explosive HE set off; afterwards, he loses his eye because of a cursed sword, then becomes a drunk when his real parents take him in after pretty much abandoning him. And it's obvious in his voicelines that he's really depressed.

Then, every Halloween whenever Medic puts his eye back in, he loses it AGAIN. And Medic, after eight years of putting his eye back, literally REMOVES a part of his brain that helps him remember things, and now he's going to have to live his life just forgetting things ("thank you kindly, stranger!" Reference to a comic panel because, LIKE, WHAT?)

In comic 7, he just looks sad when he sees the better "demoman" (it's not stated but it is heavily implied the man's something of a demoman) in both the statue and the portrait in the hallway. Or impressed, either way, he's envious of that "demoman."

He pretty much doesn't live up to his family tradition either of losing both his eyes after 30. Meaning that he was never a good Demoman. He doesn't even WANT to be a good Demoman, especially since he keeps asking Medic to put his other eye back in.

AND HE ISN'T EVEN ANGRY ABOUT EVERYTHING I JUST EXPLAINED. He has every right to be a villain, or at least an antagonist. But instead he took the sword, doesn't hold it against Medic for erasing his memory, he takes care of his blind mom even after abandoning him, and he just drinks his problems away instead of lashing out.