There's literally no reason to kill Syril at all.
He's being kept in the dark about the depth of the Ghorman operation, but is loyally and effectively doing his job. He's loyal to the Empire and to Dedra both.
There's zero reason for the Empire to clean up "loose ends." Syril can remain in the dark, it's no skin of the Empire's nose and, even if there was a danger he'd, what, flip and leak info, there's not a thing he could do to hurt the Empire's efforts.
All he knows is that the Empire is (from their point of view, rightly) keeping tabs on a rebel cell. Big whoop. What "secret" could he possibly spill that would damage the Empire in any way?
Besides, Dedra, cold as she is, isn't going to just have her boyfriend whacked for no reason.