What exactly is the whole picture? The problem here is that there are no black and white characters in the Witcher world. No one can say X is right while Y is wrong. It just doesn't work that way, or it shouldn't, in any case.
Syanna:
-Well, first off, it seems that Syanna did not wanted to use the Dettlaff from the start, i cant 100% be sure of this, Syanna said this, but obiusly i cant blindly accept everying she said as the absolute truth, but remember that she said about how they meet a few years before all this, she simply left because she was hard pressed to return those feelings, and he whould have never understood, so she left.
-What Syanna had to went trought at his age whould have screwed up anyone, just tell me, what whould have to a child if its mistreated, abused, isolated and abandoned in the middle of nowhere? We cant be use exactly the age of Syanna when all that happened but i think its about 15, thats A LOT for a kid, it whould screw up anybody.
-Syanna used Dettlaff as a tool for his revenge against the people that hurt her, and she think they hurt her (Henrietta), she could have picked 4 random innocent people to kill, she did not, remember that she aimed to kill Henrietta, the other 4 where a distraction.
-You can talk sense intro Syanna if you bother to investigate his history, so she will abandon his revenge crusade, and you can see she responds to someone that actually understood her on the fablesphere.
-Syanna returned for revenge only because she thinks her sister, and only one that helped and understanded her had abandon and betray her, Syanna has expecting Henrietta to issue a pardon and amesty for her, that did not happen, so she wrongly asumes that Henrietta forgot about it, and she will stand down if geralt can make her see she is wrong.
Ciri is not a valid comparison, Ciri never faced something like that, alone.
Dettlaff:
-He is not a child when all of this happen, first off it has emotional problems, and cant understand humans, thats already a problem.
-When he find out that Syanna tricked him in order to kill those knights, it seems to upset because a woman tricked him that is willing to destroy a city, is obiusly that he does not care about the murders, maybe only for its friend, because is willing to kill tons more. Also (and even Regis understand this), even if Syanna tricked him, thats not really a reason to kill her, since when killing a woman is acceptable just because she tricked you? And Regis supports this, and he does not have sympathy for Syanna. And dont tell me its because she trickled him to kill, Dettlaff does not gives a s*** about killing humans, the whole thing is just because she trickled him.
-When he does not get what he wants, not only he attacks a city, is willing to kill both his friend (Regis) and Geralt, someone that attempted to help him, and even showed that he was being used as a tool, he is just not thinking rationally, its acting in a brutal and impulsive way.
-He will not listen to reason that attacking a city to get what he wants is just crazy
-In the end, with ribbon is getting killed just because he thinks both Geralt and Regis tricked him, what is NOT TRUE. Again, way too impulsive, not thinking.
-He thinks that Syanna used him from the very start (since they meet), what can be or not be true, Syanna said its not, and he does not let her explain a thing.
-And i can add Dettlaff was not a just a child when all of this happen, again? its not, and its acting like one.
So, yeah, there is not a black a white here, they are both gray, but i can fully understand Syanna actions, she was really screwed up as a child, and it was out looking for revenge, and in the end, you can talk her out of it, she probably never had someone like Geralt that bother to investigate his history and ultimately maded see she was doing wrong, so she finally standed down.
Also Arturos Vigo give her a ribbon of invulneravility for a reason, he probably knewn she was not cursed and wanted to protect her.
Dettlaff had Regis help for his emotional problems, and he faced all this issue with Syanna, not only as an adult, but has a Vampire that has how many years now? and he ends up reacting in the most juvenile way possible. Also its not possible to talk out of it, he is determined to kill Syanna because of what he thinks, and thats it. Same reason of why he wants to kill Geralt and Regis, and you can see he is wrong there.
Simply put, if someone belives Syanna actions where wrong and deserved to die, so is Dettlaff, i could support that POV, but the whole thing of "Dettlaff is just a poor used Vampire, he deserves to survive", is just disturbing to me. Is so much disturbing that as this point, even Regis that wanted to save him from the start, knows that Dettlaff must die for what is done.
Also, i cant blame Henrietta for not giving up Syanna, Henrietta was willing to help the Vampire until he threaten the city, i can fully understand that, maybe if he where not going out launching threats something could have been different.