Fight him or not, Syanna is still dead. I chose to leave him alone and then found myself in prison but was later freed.
So what advantage would there have been if I had killed him?

There are 3 possible endings. You have to fight Detlaff till the death in 2, the 3rd gives you the choice to fight him or not.
The faith of Syanna and Annarietta isn't the same in all 3 endings.

Spoiler alert.














There are 3 possible endings:
1. Syanne and Annarietta both die. Detlaff dies.
2. Syanna and Annarietta both live. Detlaff dies.
3. Syanna dies and Annarietta lives. You choose whether to fight Detlaff or not.