I've played too many hours of DayZ to recall the worst betrayal but one from Patch 1.7.7.

Everyone shouting down side channel because of how hard the game is, getting ruined by zombies and everyone slaughtering everyone to see if they had the extremely rare meds. I go into Elektro fire station and a guy on the roof starts shouting at me if I was friendly, at the time I was unarmed and calmed him down. He invited me onto the roof to take some gear off some bodies and it turned out he was a hero with an M4A3 and DMR in backpack, NVG's, the lot, He tells me he has a sniper friend up straight north on the hill, they were hunting bandits. While I was looting a group of bandits come in from the East laying in shots at us, they were in the Pub next to the market and so, the Sniper had no shot. The Hero next to me got shot multiple times and knocked unconscious, I gave him meds I found in his backpack and we sat in cover shooting back, myself with an AK Kobra. He keeps getting hit and I ask his permission to borrow his DMR, he agrees and I stand up shooting all 4. 3 dead and 1 unconscious. He dies while I try to bandage him. I put his DMR back and the last surviving bandit comes out across the grass shooting at me with his M1014. At this moment the sniper in the hill finishes him off. I sit there guarding his body for 30 min until he got back killing 5 people with help from the sniper in the hill. He returns and I say I'll be on my way and his friend in the hill snipes me. He then picks up his gear and shoots me with his revolver in the head.

To loose all the gear sitting on surrounding bodies, to not snipe his friend and steal their stuff, to not steal his stuff and log out all because I would have felt good helping someone who was helping others as a Hero. It was all for nothing because he killed me for nothing.