Do you guys really think Gaunter is just some practical, sometimes malicious, chap? He murdered some drunk right in front of Geralt just because the drunk had the gal to speak his mind. God knows what else he's up to when you're not around.
In defense of O'Dimm, he does indicate twice that he'll only resort to violence if he gets annoyed or interrupted (god knows how many times that ever happens). If you look at Djinn's in the Witcher lore, they're pretty much malicious violent beings who similar to O'Dimm, grants your wishes to the word i.e. that mage that ended up dead in the ship broken in half in Last Wish. You can't exactly use the word 'evil' on Djinns because it's just part of their nature. Same way you can't actually say Succubuses are evil for driving men mad with lust, often to their deaths. A lot of negative opinions about O'Dimm revolve around him wanting to get Olgierd's soul, but not so much on the man who would sacrifice his own brother to cheat himself into becoming rich again and curse a man into a toad, and willingly agreeing to exchange his soul. It's like a heavily indebted addicted gambler cursing the bank when they try to take his house for failing to pay his loans.
O'Dimm makes no attempt to cheat Geralt and in my opinion, he didn't cheat Olgierd either. Like he mentions, it's not his fault people have crappy wishes and word them poorly. Of course there's a fear that if left unchecked, O'Dimm would continue his soul ponzi scheme and more people would get affected, but if that's the case, seeing how desperate majority people are in the Witcherverse, 4 out 10 people would be soulless. Could be wrong, but I have a feeling O'Dimm is only attracted to certain characters, probably those with despicable and cruel intentions under the guise of noble ones, and those who've given up and cursed the gods. People who mirror his own malevolent nature. I surely don't think O'Dimm is worse than the likes of Whoreson Jr or Eredin.
P.S. - In regards to the Rose Dilemma, Iris mentions she doesn't wanna continue living in the Painted World but is afraid of the endless void that death could bring. Isn't that like a general fear of death? That there could be no heaven or hell after all, just eternal darkness. She's a pretty innocent decent woman, I'm sure if there was a heaven in the Witcherverse, she would get there. Heck, even that dumb ghost in the fighting ring got into Skellige heaven after murdering a Jarl