There are two discrepancies in Part 1 that always stood out to me. One was when Makima says
Because the time has come for you to pay for your sins.
And the other one is Denji suddenly screaming out of the blue
I wanna have 5, no 10 girlfriends. I wanna have tons of sex. For that reason I wadda be Chensoman uwu
Both of these lines happen at the very finale of the story and both feel completely out of place with the rest of the story. They also come like a left flank out of nowhere, making your dazed and confused and going "wait, what?". Denji was an idealistic romantic at the start of Part 1, but now he's suddenly being painted as shallow coomer who wants 10 girlfriends so he can fuck all of them? It just felt completely out of place to me. If Denji said he wants to kill Makima because she killed Power and he needs to avenge Power, that would have been a logical motive, instead Fujimoto gives him a shallow and villainous motive.
So what did Makima mean when she said it's time for Denji to pay for his sins? That's probably something that has yet to be revealed. Either Makima knew what Denji would do in the future from Future devil, or she knew of his past sins from his previous life. Since he's a cursed soul, he's meant to be trash in every single future incarnation he goes through.
ib4 noo she meant him killing his dad
No she didn't. That's always just temporary gag bait, the reason reason for her words is something more deeper.