So, do y'all remember that first fiend that Denji encounters? Aki asks him to transform into Chainsaw Man but Denji just quickly kills the fiend with an axe. Denji gives the excuse that he sympathized with the demon, not wanting him to suffer. Aki storms off angrily or whatever, and then Denji reveals that he actually just didn't want to spill blood on the porn magazines.
The way I saw the event above was that his excuse, given to Aki, was actually the main reason for his action. He truly did sympathize with the devil. However, he lies to himself giving the porn magazine excuse instead. Like, sure, he probably did want the porn too but I don't see it as the main reason. I think he's just used to taking advantage of situations like this(as shown when he's asked to eat a cigarette for money), and that's what his mind did automatically, but he felt for the fiend too.
An event that happens later on that kind of reaffirms this interpretation is the quest for Meowy. Denji heavily sympathizes with Power's loss of Meowy, wondering how she felt whenever she would go to sleep without her cat. BUT, as soon as the Bat Devil is defeated, he makes it all seem like it was all done for fondling her breasts.
I just get the impression that, because of the way he was living beforehand, Denji's mind automatically goes to something that's "useful" for him. Deep down, though, he has other, more human, motives.