Standing inside the room and looking toward the tantrum thrown by Lucifer, who clearly wanted to murder him, Lucian felt that he should have just killed the boy at that time instead of going through all this hassle.
So he made sure to write it down in case he ever got the chance—of course, only if Lucifer survived metal poisoning.
As he turned to leave the room, he felt composed, not caring if, by chance, the plot deviated and Lucifer survived that accident a year later.
After all, either way, Lucifer was going to die in a few years on his own—not, of course, by the plot mentioned in the novel, but because that idiot didn't know that the metal ball, more specifically the bullet that hit him, was still inside his body.
From all this, Lucian learned a particular piece of information: if a bullet hits its mark, as in Lucifer's case, and he goes through healing, that healing would only regenerate his wounds but not remove the bullet from his body.
