He came to see her when it was all over, he didn't try to tell her it wasn't her fault. But he didn't scream and curse her for her mistakes either. He was human too, he knew how tempting it was to let the darkness take over. To let it swallow you whole.
They were similar…similar in a way that they others wouldn't understand. Scott was an honest, good guy. Derek, for all his faults, was a guy who wanted a family again. Lydia, human yes, but she wasn't as scared or as corrupted as they were.
Stiles could easily let someone die for the sake of his life and those he cared for, Scott would try to find every way possible to save everyone.
And that's why they fit. If pushed, they would easily let someone die before they gave up their own lies. They were damaged.
But that was okay, because they would heal. They would learn to trust again, to forgive. But for now, it would just be them.