Noble Scion
Crow, seeker of shiny things
So basically, here's what I thought so far:
1. We make human characters with their sins (I was thinking if the sins also had some embodiment or character as well)
2. The heavier the crimes committed, the stronger and monstrous the sin manifest.
3. To kill a sin, you have to help someone eliminate one of theirs. (Could they be affected by guns? And how exactly do you get rid of a sin? Does it have to do with like "Confess your sin to others" thus making your sin a bit weaker and vulnerable to eliminate) Given human nature, Confessing your sin is really hard especially if it's heavy like murder and such. In my opinion, I think it would be good to add to the plot. With this, you can explain also as to why people are scared to ask other people for help since they also have to confess their sin and would be more prone to danger.
4. I agree with @AceOfSpades0210 about his idea of this sin sudden appearance is a form of making the humans repent for their sins (A way to wipe out humanity) I think that would explain why this kind of phenomena happened. We could just explain that since humans had become so sinful, there is no better way to make them repent with their own sins. Something like that.
5. Also, does the sins only attack their owner? Or they could attack anyone close to the owner?
Noble Scion
I kinda already said 1 - 3 >.<
3. Yes, they'd be killed by guns, but guns will be so impossible to find give that they'll all have been stolen, lost or broken
4, I agree
And 5. The sins will focus the owner, but will attack others if attack
