Again, she was focused, even driven, before, but she became obsessed and made mistakes she likely wouldn't have made before. (think: her interactions with Krennic) She had always been calculating, observant and measured but losing Syril she got reckless and sloppy. She believed she was the only way to stop the rebellion and she was going to exact a toll for what had been taken from her.
A living-Syril Dedra notices someone had her log in and roots them out, a post-Syril Dedra doesn't notice. Because if that they hung her out to dry, and Partagatz with her.
From that aspect, Syril's death enabled the rebellion to win. It took so many breaks for that to happen prior to a Junior Space Wizard making an otherwise nigh-impossible shot.
