He wasn't being rewarded for his sins, instead he was thanked for bringing Kogane the divine water she needs, he was the one who brought up that he was working to atone for what he had done in the past and that he doesn't deserve thanks. Kogane basically went, keep at it, but I'm still thanking you for bringing me this divine water, and even the talk he had with Mogami after, the snacks weren't brought up as a reward for his previous sins either.He's even worse if he believes he deserves to be rewarded for his sins.
"I know apologizing won't just make everything okay," that was him saying that, does that sound like the person you seem to think he is?It doesn't seem like he'll be able to get rid of much bad karma at all. He can't even share one piece of the snacks he received with someone who's trying to help him. He's still selfish and rotten.
How about, "If I can't apologize or be hated by her, I can just quietly help with her happiness."
Does that sound like the same guy who said this in Chapter 36, "Rather than her marrying some filthy human, please let me marry Kogane-san!" ?
In your own words, him not giving a snack to Mogami must have had some malice behind it, but we have gotten some of his internal monologue through the chapter, and imho it directly counters this.
Maybe he should have offered a snack to her before she asked, maybe he should have given her one, but then again, those were snacks he got as thanks from someone who should, by all rights, despise him the moment she learns who he really was. Honestly, thinking about that way, I'm surprised Mogami even asked because of the possible impact the gesture behind the snacks would have on his psyche.
This is the Kami-sama of the same nation that categorizes Valentine's Day chocolate into two different classes to make sure the guys receiving them understand where they are on the woman's list of men in her life, whether the chocolate has genuine feelings behind them or the choco is nothing more than just an obligation.
Anyways, that's my 2c on this.