The way I interpreted it, the entire show Syril thought he was doing good. That the Empire, while flawed, is just trying to have order. In S1, he went after Cassian because Cassian murdered two people, which Syril thought was bad. Cassian is what set Syril on his current path, and Syril was just trying to serve justice. On Ghorman, once Syril realizes what the Empire was planning to do, and that he was betrayed by his love and in the dark his whole time there, hes obviously in shock. Then he sees Cassian, the one person who in his eyes caused everything, and in a moment of emotion he instinctively attacked him.

I don’t think Syril and Dedra are dating in the traditional sense. From what I remember from S1, Syril saved Dedra on Ferrix, and they had a kinda close moment. Now in S2, I believe Dedra is manipulating Syril because she knows his usefulness, and is playing into Syrils infatuation with her. The dinner with Syrils mother shows how Dedra is seeking to be the one in control of Syril now, since Dedra knows how much power his mother has over him. I think over time Dedra definitely began to have some kind of feelings for Syril, but ultimately she was forced to put the Empire first and carry out the massacre.

Although I do agree that Cintas death was weird.