In a surprising turn of events, Gabriel killed Mays in The Walking Dead season 10, episode 19, "One More," shocking viewers and Aaron. The third bonus episode to come out of The Walking Dead season 11's pandemic-related delay, "One More" focused entirely on Aaron and Gabriel as they wrapped up their two-week scavenging trip. When the Whisperers invaded Alexandria, the town was nearly razed to the ground because of the zombie horde. Thankfully, the community was in a state in which it was able to rebuild, but it's still short on supplies.
So it seems Aaron and Gabriel have been searching for food, equipment, and anything else they can take back to Alexandria. Unfortunately, although they came across a gold mine, they also came across a fiend who Gabriel killed. Mays was a survivor who had been holed up in a storage facility for what appeared to be years. Aaron and Gabriel didn't know anyone lived there when they scavenged the place, drank his whiskey, and killed and ate his boar. But when they were discovered, Mays forced them to play Russian roulette not out of retribution but because he wanted to prove that everyone is evil in the world.
When they proved him wrong, by attempting to kill themselves instead of each other, Mays stopped them and seemed to be swayed by Gabriel's ministering. They offered to take him back to Alexandria, but when Mays put his guard down, Gabriel killed him using Aaron's detached morning star arm. He said it was because Mays killed his own brother, but there's more to it. Given everything they've been through, Gabriel isn't keen on inviting a psychopathic man into their home. Plus, Mays would fail the final question they ask newcomers to the community; "Why?"
Rick Grimes would always ask three questions when interviewing new candidates: "How many walkers have you killed? How many people have you killed? Why?" The reason was to weed out the bad people - or potential bad people - before heading back home. If any answer was suspect, that person wouldn't come back. Gabriel continued to use the questions even after Rick disappeared, and Mays' explanation for what happened to his brother wouldn't have satisfied Gabriel's query - if he asked.
What's worse, Aaron and Gabriel discovered what really happened to Mays' brother; he kept him locked up in the attack, possibly for years. Not only that, but Mays' brother mentioned "playing his game," which would refer to the Russian roulette. Sadly, given the bullet holes on the two bodies in the same room - the wife and daughter of Mays' brother - it seems Mays forced the family to make a choice, resulting in two deaths. It's unclear who shot whom, or if they had shot themselves, but when the opportunity arose, Mays' brother killed himself.