Working for Thanos
Ballista Grim was a member of the Butcher Squadron, a team of killers that worked for Thanos to carry out acts of wanton murder and death. One job she was a part of was wiping out the pacifistic Dorcaernir people. Later, she and Butcher Squadron were ordered to attack the planet of Zen-Whoberi to prevent the planet from being taken over by the Universal Church of Truth.
When Thanos took an interest in a young Zen-Whoberi named Gamora, Ballista's fellow Butcher Squadron member Ebony Maw allowed Gamora to escape. When Thanos questioned the Squadron on who allowed her to escape, Ballista and the Squadron refused to talk. To keep the Squadron in line, Thanos killed one of its members, Infesti Macera.
After a mission on the planet of Sakaar, which they considered pointless, Butcher Squadron members began to question Thanos's leadership. Blood Brothers and three pilots were encouraged by the Squadron to hold Gamora for ransom. However, Gamora fought back and defeated her kidnappers. Ballista and the rest of the Squadron avoided the punishment by feigning loyalty toward Thanos.
However, they still didn't give up on their plan to betray Thanos. After Gamora threatened to tell Thanos about their betrayal, Ballista and Proxima Midnight decided to kill Gamora. The resulting chaos led to a full-scale mutiny. Thanos threatened them into quelling the mutiny and left the Sanctuary to deal with his arch-nemesis, Magus.
With Thanos gone, the Butcher Squadron once again tried to kill Gamora. However, Gamora was able to get to a ship. Ballista attempted to get Gamora out of the ship. However, Gamora took off with Ballista still on top of the ship. Ballista fell and ended up drifting in the void without a spacesuit.
In the end, Ballista settled down on the planet of Mahn Zyly. Her normal life there was, however, interrupted when one of her neighbors brought in amnesiac Nebula. Being wary of her based on their previous encounters and unaware of her amnesiac state, Ballista immediately attacked Nebula.
Presumably skilled in assassination, and she also appeared to be skilled with weapons