Around 10 years ago, I was watching Cartoon Network (Toonami) and I saw a scene from an episode of what I believe to be one of the Gundam series. In that scene, a half dozen or so older scientists were sealed in some sort of room in outer space (ship? space station?), and the oxygen was turned off, and they were left there to die. I have no idea what happened before or after that.
It sounds like episode 24 of Gundam Wing, originally aired in 1995.
Wufei, Duo, and the Gundam Engineers are in a cell, and a enemy cuts off their air supply during a coup.
Quatre, now mentally unstable, appears in the newly-constructed Wing Gundam Zero and begins to destroy various colonies one by one. Lady Une sends Heero and Trowa to fight back in the Mercurius and Vayeate, while Tsuborov, finding Une's behavior to be too lenient, stages a coup against her and cuts off the air supply to Wufei, Duo, and the Gundam engineers' cells.