"You're missing the point Harry, you are not a dark wizard now - we, all of us, carry the capability to cast the Unforgivables, the spells wouldn't have been created if it wasn't possible to cast them. And not casting them doesn't change the fact that inside of us there is the ability. And you're right, not everyone has tapped into those abilities, but it's still there. It's in James right now, it's still in you, and it's in me. You have to want to cast them, to truly want it - and I don't know that James is going to reach that point, even if he starts practicing some dark magic."
Sirius shook his head. "No, James would not say that you're a bad man, Harry. He knows that you're just trying to protect him." James may have hated the way Harry was going about doing that, but he felt confident in assuring Harry that James didn't think he was a bad man. "And that is why you're a good man, and why James will continue to be a good man. Because you both choose to be. Voldemort is the way he is, because that's what he has chosen, and you are the man that you are today because that's the path you've chosen. And James will choose to be good because he has you as an example."
He was starting to see that it was pointless to argue his point with Harry, he'd already created so many ideas in his head it would be nearly impossible for him to change them now. "I don't think it will be different in the way you imagine." He sighed. "James is determined in this Harry. And he needs help, I will help him in whatever he decides to do - but Snape will be his best ally. He knows so much more about all of the types of spells that will help Harry, but if you tell Snape not to teach them to James you will be doing your son a great disservice. He needs guidance that I can't give him."
"Do you know how stupid you sound?" Sirius asked Harry when he told him that people got killed around him. "Do you even know how many people you saved? We are in a war, you grew up in a war - people die, that's just a fact, that doesn't make it your fault. It wasn't then, and it won't be now. The blame lies with the person who cast the spell, not you."