Responses
Yes, we can't change the past. But there is atonement and forgiveness. There are people who can and can't forgive their tormenter. Perhaps the manga hasn't done a good job of portraying Sanada's regret, but we do see that he's a completely changed person now, so I don't see any problem with Itonaga being in love with Sanada. Perhaps you can't empathize with it, but I think you can sympathize :)
It dosen't matter how cute this manga is
Sanada can't change the fact that he was a bully
I mean I was a victim of bullying 4 a long time and I cant imagin my self being in love with my bully