Syril was maybe the most underrated character of the show. He actually was very complex and in a way like Dedra, was someone so focused on law and order that they didn't see that they were on the wrong side of right/wrong. They saw things as very black and white - the Empire brought order, the Rebellion was chaos. They both also didn't thing that the rules totally applied to them and that they could search and do thing on the edges (and outside the lines) so long as it was in the name of keeping order in the galaxy.
Where I think his major failing (as an Empire lacky) was, and what really kept him on the good side, was his origin. Dedra found a way to get all the way to the top of the stack, working her way up from basically nothing, but remember she was in the system the whole time, she was drinking the kool-ade from day 1. Syril's lack of means found him outside the system for so long early that when he finally got in, he was a believer, but he had a sense of right and wrong that wasn't fully shaped by the Empire. Plus he entered in from so far down the ladder, if he was going to make it to the top, he was going to need everything to go perfect - and his failure to get Andor in S01E03 derailed that from the start.
He became tragic because:
He wanted Andor so bad, he was his white whale. He hunted for him all of season 1 and Andor always eluded him
In the end he realized he was on the wrong side but was powerless to stop it and his pursuit of Andor pushed him back to the other side right at the end
The "Who are you?" was also so tragic because he hunted Andor so much and knew him so well, while Andor met him once and probably never got a look at his face (or cared since he was trying to escape with Luthen)
Last thing on Dedra-Syril, very interesting how their plots intertwined and were parallels. Both had a target they wanted: Syril-Andor, Dedra-Luthen (Axis) and finally "getting" them was their ultimate downfall when they finally met them at the end.
One last thing on Syril - I really was hoping that he would figure it out and flip during this season. I was going to post pre-season 2 that I thought he would flip to the rebellion. I think he could have and his arc could have done that had he not seen Cassian on Ghorman.