What fate had done, what Alex himself had predicted. But no, Nick couldn't deny his culpability and he didn't believe in fate except for what the kid himself had proven to him. He could get why Alex was pissed; he had killed him, there was no getting around that fact. It wasn't like Nick hadn't gone over it in his head a thousand times, wondering if there was some other way (knowing there was always another way). But you couldn't change the past.
"I did what had to be done, for the greatest good," he said, unflinching. Lying or a false apology wasn't going to get him anywhere. Alex knew him well enough to know if he was insincere and he was sure Callista would pay the price if he wasn't honest. "I'm not even going to say it was 'right' thing to do, but at the time it was necessary. Your blood is on my hands, but it's not like either of us can change that, so I don't know what it is you want from me here, kid."