Interfering when Gaunter O'Dimm goes to collect Olgierd Von Everec's soul and beating the Master Mirror at his own game is generally considered the good ending of The Witcher 3's Heart of Stone expansion. That sort of chivalry is certainly inline with Geralt's white knight mentality. But after experiencing Olgierd's backstory, referring to lifting his curse as "Good" has always given me a bit of pause. So today, I am here to make a case for why letting Olgierd die is, in fact, the lesser evil.
It's not that O'Dimm can be justified. It's apparent that the Master Mirror is a demon and by trade, Geralt should feel compelled to dispatch the threat he poses. Though if there's anything to be said for Gaunter O'Dimm, it's that many believe he simply mirrors the evil that already dwells within those he strikes contracts with. Olgierd was always an unsavory character who already demonstrated cold inhumanity and unempathetic qualities. It could be said that O'Dimm merely magnified the inhumanity which Olgierd already possessed.
Olgierd's Backstory
As a young man, long before encountering Gaunter O'Dimm, Olgierd formed a gang of noble-born bandits with his brother. This means the Von Everec brothers and their high-born rabble chose to engage in raiding and pillaging for shear fun, not of any hard-luck or necessity.
Despite his deplorable morals, Olgierd managed to win Iris's affections, a noblewoman, and they fell in love. 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.
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 in order to retain his wealth and relationship with Iris. Olgierd also wished to "live like there was no tomorrow," which O'Dimm effectively twisted to mean immortality with a heart of stone.
As Olgierd began to feel less and less, he grew distant from Iris, and his actions became more and more deplorable. He begins practicing Goetia in attempts to reverse his dealings with O'Dimm, winds up killing Iris' father when she enlists him to end their marriage, then leaves Iris in pursuit of rediscovering some excitement in life. Excitement which he finds in his favorite pastime of raiding and pillaging. Though he explains his reasons for leaving were to spare Iris any more pain, it wasn't out of love or care for her. Moreso, the memory of the love he had once felt.
Some may defend these actions, stating Olgierd committed these acts because of O'Dimm's curse, and yes, he did try to reverse the effects. But the fact of the matter is, even before the curse and his failing marriage, Olgierd had a lifetime of blood on his hands, including his own brother's.
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. 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 Geralt lifts the curse, Olgierd says himself the victory is bitter, and he seems morose about continuing on in life.
Of course, Geralt encourages Olgierd to press on and start anew, saying he still has companions that support him and wealth to his name. Olgierd does promise to start a new life, but the sad fact of the matter is though a snake may shed its skin, it can't change its nature. With an inborn lack of humanity and a predilection for violence, which he indulged in his entire life, I find it doubtful Olgierd would be able to turn over a new leaf.
Olgierd may have learned a lesson throughout his ordeal with O'Dimm, but likely more on the front of bargaining with his soul, not rethinking his immoral lifestyle. After his heart is restored, his sense of hopelessness could drive anyone back into bad habits, especially ones they never fully kicked. And those companions Geralt reminds him about are a pack of bandits who supported Olgierd as the leader of their raids. With that kind of support system, it's doubtful Olgierd is capable of redeeming himself even if he does make an honest attempt.
Inadequate Compensation
If you're torn at all between who Geralt should side with, then let's break it down to a matter of compensation. From a gamer's perspective, Olgierd's reward simply cannot compare with the rewards Gaunter O'Dimm has to offer.
After collecting Olgierd's soul, the Master Mirror offers Geralt a choice from several exceedingly useful rewards. These include an infinite food supply- meaning infinite vitality-, and bottomless vodka for limitless alchemical creations, among others. He even offers detailed advice on parenting Ciri, which leads to achieving her witcher outcome in the endgame.
Conversely, if you save Olgierd and convince him to start a new life, he thanks you by handing over his sword. The weapon isn't without its merits as it's magically imbued and can deal massive damage once fully charged (costing the wielder a chunk of vitality as well). 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. Either end has the same outcome in that respect, so it's a matter of choosing a lesser evil when deciding which one to let walk free. But if you pity Olgierd, letting him die mercifully allows him to escape his grief and ensures he won't continue harming innocents.