There are plenty of these moments during the show and tie-in media.
- In season 4 Garibaldi has another personality created in his brain by Bester. He ends up selling out his friends and betraying everything they stood for while his real self is watching helplessly.
- This is implied with Sinclair/Valen. After he went back in time and became Valen he left letters for the future. He couldn't do anything to stop the Earth-Minbari war (which he had fought in before going back in time) in case that would drastically change things and give The Shadows an advantage. We never find out what would've happened, nor does he.
- The second Narn-Centauri war has plenty of this. Vir, who was against it long before it started, is sent to bring Mr Morden to Londo for the Deal with the Devil that starts it. After the fall of Narn he tries to apologize to G'Kar and is rejected. As the Centauri become more genocidal he becomes The Atoner by smuggling 2000 Narn to safety. Meanwhile Sheridan and Delenn are stuck with governments who either will not intervene or become allies with the Centauri. Londo eventually has a Heel Realization.
- The Drakh can do this with their keepers. They forced The Regent of Centauri Prime to turn off the planetary defense grid just as a big fleet was about to attack. They later turn Londo into a Puppet Emperor by threatening him with blowing up the nukes they have hidden around the planet.
- In The Legions of Fire trilogy they detonate one third of those bombs after Londo attempts to call their bluff. It turned out that the resistance hadn't disarmed them as they didn't know they existed. Londo is left remembering what Elric had told him long ago. "Your victims."
- Lennier has a Moment of Weakness and leaves Sheridan to die. When he comes back to help Sheridan has already gotten himself out of the dangerous situation.
- Delenn believes that all life is sacred. In late season 2 she, accompanied by Lennier, goes into the Markab isolation zone to offer comfort to the sick. When the station personnel open the door after they've found the cure everyone except Delenn and Lennier are already dead. What makes it worse is that the whole thing could've been avoided if the Markab had asked the other worlds for help when they started falling ill a year earlier. The Markab doctor we see is implied to have had a My God, What Have I Done? moment about listening to the leaders and keeping it secret. He then lets Franklin study how the disease progresses in him in hopes of finding a cure.
- Delenn had another moment before the series started. She's the one who cast the deciding vote for the Earth-Minbari war when Dukath was killed in the first contact with the humans. The humans weren't immune to this either. The Captain on the Prometheus panicked when the Minbari scanners disabled their jump engines and ordered his crew to open fire. Delenn later meets the Gunnery Sergeant who fired the first shots and is also dealing with this trope.
- In "Passing Through Gethsemane," peaceful monk Brother Edward finds out he used to be a serial killer before his personality was replaced, and his victims have tracked him down to make him suffer and remember.
- Dr. Franklin performs a "simple" surgery on an alien child against the wishes of the child's parents, who want to allow their child to die. His decision backfires; their religion believes opening the body releases the soul, and the horrified parents kill what they perceive as the empty shell that once housed their child.
- Jinxo/Thomas refuses to leave Babylon 5 after the previous 4 stations were sabotaged/disappeared right after he left, believing that their losses were somehow caused by leaving.
- Dr. Rosen becomes The Atoner after she got addicted to stims and made a mistake that killed a patient. She develops an incurable disease, but finds meaning from using an alien healing machine to transfer her life force to others. And then she's faced with a murderer threatening to kill her daughter if she won't heal him with the machine, so she instead reverses it. Her disease is cured, but she has taken another life.
- Garibaldi prides himself on being in control of whichever situation he's in and being good at reading people. Then his second in command shoots him in the back.
- Talia goes through this when she starts to realize that the Psi Corps isn't as good as she was raised to believe, and then Lyta returns to the station with news of a Psi Corps sleeper agent. When the sleeper agent is activated, the host personality will be erased and replaced. The sleeper agent is Talia, and she essentially dies while the new personality lashes out at the people Talia loved.
- By the end of season 4 when Marcus makes a Heroic Sacrifice for Ivanova, who never admitted having mutual feelings until it was far too late.
- Jolly, helpful, hand-wringing Vir killing Cartagia to put an end to the evil.
