Wyatt Chirp, may he rest in peace. Hey, Blind Mew!
The Magnificent Seven broke up after the death of Wyatt Chirp, but how and why he died is never actually said. We know the following:
Somehow, Duck Holiday is complacent in it, but we don't know how. All we know is Buffalo Bill blames him for the death of Wyatt Chirp and the rest don't care much for him.
He was apparently killed by the Wild Bunch, but under what circumstances, we do not know.
A pair of Peacemakers were involved, again, we don't know how.
Re: Wyatt Chirp, may he rest in peace. Hey, Blind Mew!
Old man Bronco in the Old Jailhouse seemed to hint that he had something to do with Wyatt Chirp's death, or at least knew of what happened. He says something along the lines of "It's time for you to learn what happens to heroes - Just like Wyatt learned". I learned this recently while questing on my swashbuckler.
I've always been very curious about this, I really hope Mew gives us some sort of answer.