Ah, wow, I didn't pay that close attention to the tower quest (and only started it so far). That's even better that more is coming. I guess the DRM thing caught my attention so much that it blinded me for the GoG.
The letter, I guess, could be on his body if you kill him. I let him live and he told me the location of his stash and there I found it.
Left me with slight mixed feelings. I assumed he gave me his savings (money seeming so mortally important to him) as a bit of penance for what he did, self-discipline. And the letter is further reason for him being in emotional turmoil.
He did have three trophies hanging on the wall, and Geralt commented that maybe they tried to scam him about those, too. So either he's got a hoarder problem or something or a streak of bad luck.
I let him live because he was open and sincere about what he did, and I respect that. I also very much do not respect rotten character. The people not knowing how to pay him was no excuse for filthy murderous betrayal. It was an opportunity for them to show some decency and figure something out, like others did, but they decided to be weak and give into wickedness, and that's always a personal decision that responsibility needs to be taken for.
As for whether the other people there knew/agreed, that's debatable, but due to own real life experiences I know that few people are actually as "innocent" as Geralt called them.
The fact that even in his vengeful fury the witcher spared the child showed that it was indeed fueled by rage over that rotten character. He instinctively knew a child would be innocent.
I didn't tell the woman the truth. I said 'something like bandits, yeah'. Easy decision after just having witnessed the weakness in commoners.
BTW I also spared Letho in W2. I was just so damn tired and exhausted from all the drama and bloodshed and Letho seemed to feel the same. (How immersive! And also shows that the ultimate solution to such is to let go of the mind's control and its concepts. To just stop and breathe. To no longer buy into the madness.)