WARNING: The following contains spoilers for Boruto: Naruto Next Generations Episode 182, "Ao," now streaming on Crunchyroll.
In Naruto and Boruto, many ninja have evolved from villains to heroes. Sasuke stands out as a Ranger making amends for his own sins and that of the Uchiha clan, while former big bad Orochimaru is now supposedly running experiments to better society and encouraging Log and Mitsuki to do good. However, there are others who went in the opposite direction and regressed into the darkness. One of these is Ao, who was recently exposed as a spy for Kara. His fall from grace has been truly tragic.
Ao was always one of the deadliest ninja. In his encounter with Shisui Uchiha, an elite Konoha soldier himself, Ao proved his worth, whether it was by weaving one-handed seals or using his Byakugan. He actually stole and transplanted this eye after a battle with a Hyūga clan member from Konoha, which isn't an easy thing to accomplish. It would garner him respect and fear, which ultimately saw him protecting Kirigakure's Mizukage when the time came for all nations to unite against Madara, Kaguya's puppet.
This is where Ao's outlook on life started to change drastically. He saw through Danzo's scheme and realized he was creating havoc as Ao could have spotted the aura of Shisui's Sharingan on the fiend. He could tell Danzo was transplanting eyes of his own and he had a role in Sasuke breaking bad. It proved him why he couldn't trust villages like Konoha, but while Ao's squad fought Danzo and Fu (one of the Yamanaka clan in Konoha), they had to switch gears to Madara's army and the Great Ninja War.
Still, as much as had to work with the likes of Konoha, Ao always harbored cynicism. It culminated in the Ten Tails Beast dropping a bomb on Ao's new unified team, which included Ino's dad and Shikamaru's dad. However, they sent info back for Naruto to save the day just before they died and while Ao's body was recovered from the rubble, he'd step back from war. He was in a coma for years, nursed by Katasuke and Konoha's scientific experts, and this changed him for the worst.
Losing soldiers time after time didn't have a good effect at all. It seemed like Ao became more reserved, as seen when he visited Konoha and paid respect to the fallen on the anniversary of the war, but all this time, Ao was a double agent -- losing his humanity the more parts of him were replaced by machinery. He was the one who placed a genjutsu on Katasuke to steal data for Jigen and Kara and has secretly been Koji Kashin's war dog. Now he's tasked with hunting down the Vessel that Kara lost.
Despite offering advice to Boruto and co. on helping contribute to society, Ao was playing them all. He didn't care for Konoha or its kids anymore as he saw them as part of the problem -- always producing a Sasuke or Danzo. Ao lost compassion and empathy, believing Kara, much like the Akatsuki, needed to wipe the world clean. As he went hunting the Vessel, collateral damage wasn't even on his mind. He was willing to give his life and take down people he once worked with to create this utopia, which is why Kara knew he'd be ideal for their suicide missions.