Light Yagami/Harry Potter
Light's nightmares didn't take nationality into consideration. They didn't take his rebirth into consideration. They certainly didn't take the bitter taste at the tip of his tongue, the harrowing cold in his heart and the stone in his stomach into consideration.
They just came.
Light's killing of Lind L Taylor. The trap in his desk that could have killed Sayu. The trap L had set for him post mortem that did lead to his father's death and his sister's trauma some six years later after Light had written them off as unimportant. His mother's grief. Matsuda, that idiot, the only one who had asked him for answers after Light had been exposed as Kira.
Light did not regret. It went against his very being. He was a sociopath. Light knew he was a borderline when he was 17. That was why Ryuuzaki's taunts had hit as hard as they did, why his denials had been loud. He knew he had it in him to become Kira. Had acknowledged it just a few months prior. Adult him was a narcissistic megalomaniac sociopath with a God-complex and all the tools to pull it off. Death Notes really don't help with curbing superiority complexes.
But his mind, his new body-!
No, Light Yagami did not regret his actions one bit. But thinking back, there were a lot of things he should have done differently.