[Blood and Wine Spoilers ahead]
I know this is a very unpopular opinion. Nevertheless, would like to share why I think this.
Syanna planned the death of a handful, manipulated detlaff to seek revenge. Her motives, understand them or not, might not justify her means. The key point for me here is the number of deaths planned.
Detlaff though initially manipulated, later decided to murder 100s of innocents on his own will. His being more emotional than normal can explain this among other things but does not justify this. He was solely responsible for these deaths.
Anna chose to trade the life of her sister, even after learning she was responsible for the murders, for the lives of 100s of innocent civilians whom she could have saved. It was just plain stupid for a ruler who claims to care so much for her people.
Having said this I would like to admit that I do understand the hate that syanna gets, and respect the opinions. But even after 2 playthroughs, I simply don’t agree.