All you have to do is either choose the option "Don't get involved" when O'Dimm attempts to kill Olgierd or not meet with Shani and the professor. Regardless of which method you pick, Master Mirror murders Olgierd. It's among the most tragic character endings in the game, but at least you receive a reward for it.

What happens if you don't help Olgierd?

There you will complete the last Olgierd's objective and will reach the ending. If you have completed the side quest of meeting with Shani, you will have a choice: pick the side of Gaunter O'Dimm's or Olgierd's. If you haven't completed the quest, the battle will be automatically won by O'Dimm.

Is Gaunter O DIMM a bad guy?

Gaunter O'Dimm, also known as Master Mirror or The Man of Glass, is a character from The Witcher 3: Wild Hunt. In the main game, he originally appears as a minor character. However, he returns as the main antagonist in the DLC Hearts of Stone.