I don’t think Xebec will actually kill Eris. All D’s die with a smile on their face after all
It’s not really a D thing, it’s most flashback characters go out with a smile as a last show of defiance. Bellemere, Hiriluk, Oden, Brook’s crew are all non D’s who did the same, while Law’s family were certainly not smiling, and I doubt Clover and his brother was either.
If Xebec manages to pull himself out of the Domi Reversi and get a moral victory over Imu, he’ll die smiling. If he’s too go out in bad circumstances to set up Blackbeard’s rise to villainy, he won’t.