(Timestamped) It’s in reference to the event on screen occurring as he’s saying that line.
Lord Saladin: Before they forced us to lift up a murderer as our savior.
This “Murderer” is Rasputin: the only one who could take down the Almighty within the time we had during its descent on the Last City. Osiris also mentions this when he confronted Rasputin right before Season of the Worthy.
Osiris: It’s been a while, old man. The Tower looks at you and sees a god. But I see a thug. A murderer. Betrayer. You know why I left the Infinite Forest. What I saw. A line has been drawn in this system. Light on one side. Dark on the other. Where do you stand?
Eris Morn also explains this to the Drifter as well during Prophecy.
Eris Morn: Rasputin?
Drifter: That’s his name. Osiris was bankin’ on him. Kicked down that old Russian’s door and threatened him with a gun to get him to help. He didn’t know I was watching. And laughing. Heh.
Eris Morn: There’s no humor in that. Rasputin murdered the Iron Lords. Stayed in the shadows to save himself as humankind burned.
On the off chance you’re newer to the story (not played D1 at all), check out the opening cutscene from Rise of Iron where SIVA is killing the Iron Lords along with Iron Lords 2.5 and of course, The Tyrant mission.