First of all, this DLC is a minimized version of Faust, the world famous literature work of Wolfgang von Goethe.

Second, I have worked as volunteer psycho analyst, went through minds, thoughts, dreams, nightmares of other people. Some parts of this DLC, feel exactly like the process of psycho analysis: you dive deep into the mind of others, collect the pieces of their memory they tried to avoid, reassemble their stories, and get them out of the trap.

Third, I wanted to do the same as you at the very beginning of the DLC, I felt like I would 100% killed him if I got a chance, right after he cut a sculpture into half. But after I saved him, I found it is worthy of my effort, not only because of Olgierd appreciation, but more importantly, because this deed strenghtened myself: by risking my in-game life to save Olgierd, a person who seemed unredeemable, I can believe when I fall into the traps of devils make terrible mistake and commit terrible crimes, there will always be someone out there who will risk their lives to save me.

Also, if you save Olgierd, and get the best endings in other two parts, the whole Wiitcher 3 feels more like a magnificent trilogy instead of a mixture of stories: In the first part, you sacrifice your emotion for a better future beyond your scope of experience(Empress Ciri); in the second part, you can see your actions have given a character a new life, but yet not experience the things afterwards; in the last part, you are rewarded with the experience of fairy tale-like world after the whole story, forged by your actions.

At last, the boss fight against O'Dimm is unique, and a challenge of intelligence and preception. Even you hate Olgierd that much, you should save him, just for the boss fight.