"But that's just my point," Harry said. "I didn't learn to cast the Cruciatus by practising the Unforgivables, but by being exposed to Dark magic every day for a period of time. I don't even know when I changed enough, when I was sufficiently dark to do it. I just did it."
"That's what training does. He won't change over night, but every time he practise he'll change a tiny bit. So when in that process do we know that it's too much? That if he keeps going he'll be able to look someone in the eyes and cast the Cruciatus without hesitating? We won't know, James won't know, not until it's already too late."
He looked down when Sirius said he wasn't a bad man. "I'm not sure James would agree with you," he said, pausing. "I try to be a good man, I've made that choice. I've made it consciously every day for over 25 years. But that doesn't mean my soul isn't damaged, that it isn't a struggle not to take the easy way out."
"The killing, should it come to that, will be one time. It's one spell. It will change him, yes, there is no denying that, I'm not denying that. It's still different. He won't be exposed to it the way training will do."
He smiled. "Besides, as far as best possible chances go, training to adapt what he already knows rather than training him in completely new forms of magic, so different from everything he knows, is likely much better. Everything I know about learning tells me that."
He shook his head and smiled. "The only one I've been talking to is my son. He pointed out that I get people killed, and as much as I wish he was wrong he's not. I do. I'm glad you don't blame me, but it was my fault. I will always live with that regardless. Not only your life but other's as well. James is right, it's dangerous to know me. Do you think he'd be in this mess if he wasn't my son? I can't even keep my children safe."