While playing the game I heard a few dialogues between Reaper and Mercy
Mercy: "This is not what I intended for you, Reyes."
Reaper: "You knew exactly what you were doing."
Mercy: "What happened to you?!"
Reaper: "You tell me, Doc."
I'm intrigued, what did Mercy do that gave Reaper so much hate
Best Answer
I think it's hinted at (if not outright confirmed) in Overwatch lore that Mercy was the one who was responsible for Reaper's current condition after he was apparently killed at Overwatch HQ. Her attempts to revive him didn't go as planned, and rather than being restored as the Gabriel Reyes everyone knew, his cells started to destroy and regenerate themselves in a very rapid manner, which is what causes him to sometimes appear as a black fog. This is what allows him to assume his "wraith" form and allows him to teleport.