That sort of chivalry is certainly inline with Geralt’s white knight mentality.
It’s not that O’Dimm can be justified.
Olgierd was always an unsavory character who already demonstrated cold inhumanity and unempathetic qualities.
Edit via witcherhour.com & youtube.com
It could be said that O’Dimm merely magnified the inhumanity which Olgierd already possessed.
But through a series of unfortunate events, the Von Everec family lost their wealth and societal ranking.
This put Olgierd and Iris' planned marriage in jeopardy.
Via sportsskeeda.com
About this time, Olgierd first meets O’Dimm and begins bargaining to reverse his family’s ill fortunes.
He was quick to sacrifice his brother to make it retain his wealth and relationship with Iris.
Excitement which he finds in his favorite pastime of raiding and pillaging.
Via witcher-wiki.fandom.com
Moreso, the memory of the love he had once felt.
A Tiger Can’t Change His Stripes.
Stepping aside and letting Olgierd perish at the hands of Gaunter O’Dimm is not about condemning Von Everec.
Via guidesforgamers.com
After everything Olgierd has been through, it should be viewed as a mercy killing.
He has suffered long and has lost everyone he cared for, albeit due to his own self-serving actions.
Regaining his heart is no reward when the only emotions that wash over him are grief and regret.
After collecting Olgierd’s soul, the Master Mirror offers Geralt a choice from several exceedingly useful rewards.
He even offers detailed advice on parenting Ciri, which leads to achieving her witcher outcome in the endgame.
However, Geralt will out-level this weapon just like any other within five or so levels.
This makes some of O’Dimm’s potential rewards more valuable and far longer lasting.
To recap, lifting Olgierd’s curse makes him miserable, and Geralt is poorly compensated for his efforts.
The only good outcome of this ending is that Geralt successfully vanquished another monster.
Yet, he potentially allowed another to continue their ways.
